Package build with GCC 9.3.0 and CMake (not pinned) not results in dependency error.

- [x] I read [the conda-forge documentation](https://conda-forge.org/docs/user/introduction.html#how-can-i-install-packages-from-conda-forge) and could not find the solution for my problem there.

Issue:
When trying to build a package that include

- gcc 9.3.0, e.g. via `{{ compiler("c") }}` + build variant file
- cmake (no version pinned)

the `conda build` command will results in a dependency error while resolving the *build* environment.

I attached an example, including a .sh file for quick execution:
[gcc-test.tar.gz](https://github.com/conda-forge/conda-forge.github.io/files/7093097/gcc-test.tar.gz)

It seems that it somehow fails to select the correct version of CMake.
According to `conda render` it selects cmake 3.21.2, which requires gcc>=9.4.
It *should* select cmake 3.21.1, which still works with gcc=9.3.

Environment (conda list):

```
$ conda list
# packages in environment at /home/user/miniconda3:
#
# Name Version Build Channel
_libgcc_mutex 0.1 conda_forge conda-forge
_openmp_mutex 4.5 1_gnu conda-forge
attrs 21.2.0 pyhd8ed1ab_0 conda-forge
backports 1.0 py_2 conda-forge
backports.functools_lru_cache 1.6.4 pyhd8ed1ab_0 conda-forge
beautifulsoup4 4.9.3 pyhb0f4dca_0 conda-forge
bitstruct 8.11.1 pypi_0 pypi
boa 0.6.2 py38h578d9bd_0 conda-forge
brotlipy 0.7.0 py38h497a2fe_1001 conda-forge
bzip2 1.0.8 h7f98852_4 conda-forge
c-ares 1.17.2 h7f98852_0 conda-forge
ca-certificates 2021.5.30 ha878542_0 conda-forge
certifi 2021.5.30 py38h578d9bd_0 conda-forge
cffi 1.14.6 py38ha65f79e_0 conda-forge
chardet 4.0.0 py38h578d9bd_1 conda-forge
charset-normalizer 2.0.0 pyhd8ed1ab_0 conda-forge
colorama 0.4.4 pyh9f0ad1d_0 conda-forge
commonmark 0.9.1 py_0 conda-forge
conda 4.10.3 py38h578d9bd_0 conda-forge
conda-build 3.21.4 py38h578d9bd_0 conda-forge
conda-package-handling 1.7.3 py38h497a2fe_0 conda-forge
cryptography 3.4.7 py38ha5dfef3_0 conda-forge
dataclasses 0.8 pyhc8e2a94_3 conda-forge
diskcache 5.1.0 pypi_0 pypi
filelock 3.0.12 pyh9f0ad1d_0 conda-forge
future 0.18.2 py38h578d9bd_3 conda-forge
glob2 0.7 py_0 conda-forge
icu 68.1 h58526e2_0 conda-forge
idna 3.1 pyhd3deb0d_0 conda-forge
importlib-metadata 4.8.1 py38h578d9bd_0 conda-forge
importlib_metadata 4.8.1 hd8ed1ab_0 conda-forge
inotify_simple 1.3.5 py38h578d9bd_1 conda-forge
jinja2 3.0.1 pyhd8ed1ab_0 conda-forge
joblib 1.0.1 pyhd8ed1ab_0 conda-forge
json5 0.9.5 pyh9f0ad1d_0 conda-forge
jsonschema 3.2.0 py38h32f6830_1 conda-forge
krb5 1.19.2 hcc1bbae_0 conda-forge
ld_impl_linux-64 2.36.1 hea4e1c9_2 conda-forge
libarchive 3.5.2 hccf745f_0 conda-forge
libcurl 7.78.0 h2574ce0_0 conda-forge
libedit 3.1.20191231 he28a2e2_2 conda-forge
libev 4.33 h516909a_1 conda-forge
libffi 3.3 h58526e2_2 conda-forge
libgcc-ng 11.1.0 hc902ee8_8 conda-forge
libgomp 11.1.0 hc902ee8_8 conda-forge
libiconv 1.16 h516909a_0 conda-forge
liblief 0.11.5 h9c3ff4c_0 conda-forge
libnghttp2 1.43.0 h812cca2_0 conda-forge
libsolv 0.7.19 h780b84a_5 conda-forge
libssh2 1.10.0 ha56f1ee_0 conda-forge
libstdcxx-ng 11.1.0 h56837e0_8 conda-forge
libxml2 2.9.12 h72842e0_0 conda-forge
lz4-c 1.9.3 h9c3ff4c_1 conda-forge
lzo 2.10 h516909a_1000 conda-forge
mamba 0.15.3 py38h2aa5da1_0 conda-forge
markupsafe 2.0.1 py38h497a2fe_0 conda-forge
ncurses 6.2 h58526e2_4 conda-forge
openssl 1.1.1k h7f98852_1 conda-forge
patchelf 0.13 h58526e2_0 conda-forge
pip 21.2.4 pyhd8ed1ab_0 conda-forge
pkginfo 1.7.1 pyhd8ed1ab_0 conda-forge
prompt-toolkit 3.0.8 pypi_0 pypi
prompt_toolkit 3.0.20 hd8ed1ab_0 conda-forge
psutil 5.8.0 py38h497a2fe_1 conda-forge
py-lief 0.11.5 py38h709712a_0 conda-forge
pycosat 0.6.3 py38h497a2fe_1006 conda-forge
pycparser 2.20 pyh9f0ad1d_2 conda-forge
pygments 2.10.0 pyhd8ed1ab_0 conda-forge
pyopenssl 20.0.1 pyhd8ed1ab_0 conda-forge
pyparsing 2.4.7 pypi_0 pypi
pyrsistent 0.17.3 py38h497a2fe_2 conda-forge
pysocks 1.7.1 py38h578d9bd_3 conda-forge
python 3.8.10 h49503c6_1_cpython conda-forge
python-libarchive-c 3.1 py38h578d9bd_0 conda-forge
python_abi 3.8 2_cp38 conda-forge
pytz 2021.1 pyhd8ed1ab_0 conda-forge
pyyaml 5.4.1 py38h497a2fe_1 conda-forge
readline 8.1 h46c0cb4_0 conda-forge
reproc 14.2.3 h7f98852_0 conda-forge
reproc-cpp 14.2.3 h9c3ff4c_0 conda-forge
requests 2.26.0 pyhd8ed1ab_0 conda-forge
rich 10.9.0 py38h578d9bd_0 conda-forge
ripgrep 13.0.0 habb4d0f_0 conda-forge
ruamel.yaml 0.17.16 py38h497a2fe_0 conda-forge
ruamel.yaml.clib 0.2.2 py38h497a2fe_2 conda-forge
ruamel_yaml 0.15.80 py38h497a2fe_1004 conda-forge
setuptools 57.4.0 py38h578d9bd_0 conda-forge
six 1.16.0 pyh6c4a22f_0 conda-forge
soupsieve 2.0.1 py38h32f6830_0 conda-forge
sqlite 3.36.0 h9cd32fc_0 conda-forge
tk 8.6.11 h21135ba_0 conda-forge
tqdm 4.62.2 pyhd8ed1ab_0 conda-forge
typing_extensions 3.10.0.0 pyha770c72_0 conda-forge
urllib3 1.26.6 pyhd8ed1ab_0 conda-forge
wcwidth 0.2.5 pyh9f0ad1d_2 conda-forge
wheel 0.37.0 pyhd8ed1ab_1 conda-forge
xz 5.2.5 h516909a_1 conda-forge
yaml 0.2.5 h516909a_0 conda-forge
zipp 3.5.0 pyhd8ed1ab_0 conda-forge
zlib 1.2.11 h516909a_1010 conda-forge
zstd 1.5.0 ha95c52a_0 conda-forge
```

Details about conda and system ( conda info ):

```
$ conda info

active environment : base
active env location : /home/user/miniconda3
shell level : 1
user config file : /home/user/.condarc
populated config files : /home/user/.condarc
conda version : 4.10.3
conda-build version : 3.21.4
python version : 3.8.10.final.0
virtual packages : __linux=5.8.0=0
__glibc=2.31=0
__unix=0=0
__archspec=1=x86_64
base environment : /home/user/miniconda3 (writable)
conda av data dir : /home/user/miniconda3/etc/conda
conda av metadata url : None
channel URLs : https://conda.anaconda.org/conda-forge/linux-64
https://conda.anaconda.org/conda-forge/noarch
https://repo.anaconda.com/pkgs/main/linux-64
https://repo.anaconda.com/pkgs/main/noarch
https://repo.anaconda.com/pkgs/r/linux-64
https://repo.anaconda.com/pkgs/r/noarch
package cache : /home/user/miniconda3/pkgs
/home/user/.conda/pkgs
envs directories : /home/user/miniconda3/envs
/home/user/.conda/envs
platform : linux-64
user-agent : conda/4.10.3 requests/2.26.0 CPython/3.8.10 Linux/5.8.0-63-generic ubuntu/20.04.3 glibc/2.31
UID:GID : 1000:1000
netrc file : /home/user/.netrc
offline mode : False
```