Error only when building a Fully Statically Linked Binary on Linux

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I'm using PyOxidizer to build a binary for my project [Cifra](https://github.com/dante-signal31/cifra) (at my "pyoxidizer" branch).

My pyoxidizer.bzl is at root folder, next to setup.py:

```python
def make_exe():
dist = default_python_distribution()
policy = dist.make_python_packaging_policy()
policy.resources_location = "filesystem-relative:lib"
python_config = dist.make_python_interpreter_config()
python_config.run_command = "from cifra.cifra_launcher import main; main()"
exe = dist.to_python_executable(
name="cifra",
packaging_policy=policy,
config=python_config,
)
exe.add_python_resources(exe.setup_py_install(package_path=CWD))
return exe

def make_embedded_resources(exe):
return exe.to_embedded_resources()

def make_install(exe):
files = FileManifest()
files.add_python_resource(".", exe)
return files

def register_code_signers():
if not VARS.get("ENABLE_CODE_SIGNING"):
return

register_code_signers()

register_target("exe", make_exe)
register_target("resources", make_embedded_resources, depends=["exe"], default_build_script=True)
register_target("install", make_install, depends=["exe"], default=True)

resolve_targets()
```

With that configuration I get a proper binary file with *"pyoxidizer build"*.

However, problem arises when I try to build a fully statically linked binary. I've followed [instructions page build static binaries](https://pyoxidizer.readthedocs.io/en/latest/pyoxidizer_packaging_static_linking.html). I've updated rust:

```bash
(venv)dante@Camelot:~/Projects/cifra$ rustup target add x86_64-unknown-linux-musl
```
But when I build process ends with this error:

```bash
(venv)dante@Camelot:~/Projects/cifra$ pyoxidizer build --target-triple x86_64-unknown-linux-musl
[...]
Running greenlet-1.1.1/setup.py -q bdist_egg --dist-dir /tmp/easy_install-futc9qp4/greenlet-1.1.1/egg-dist-tmp-zga041e7
no previously-included directories found matching 'docs/_build'
warning: no files found matching '*.py' under directory 'appveyor'
warning: no previously-included files matching '*.pyc' found anywhere in distribution
warning: no previously-included files matching '*.pyd' found anywhere in distribution
warning: no previously-included files matching '*.so' found anywhere in distribution
warning: no previously-included files matching '.coverage' found anywhere in distribution
error: Setup script exited with error: command 'musl-clang' failed: No such file or directory
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Custom { kind: Other, error: "command [\"/home/dante/.cache/pyoxidizer/python_distributions/python.70974f0c6874/python/install/bin/python3.9\", \"setup.py\", \"install\", \"--prefix\", \"/tmp/pyoxidizer-setup-py-installvKsIUS/install\", \"--no-compile\"] exited with code 1" }', pyoxidizer/src/py_packaging/packaging_tool.rs:336:38
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

To make it work, I've installed musl, musl-dev an clang packages but error persists. I'm in a Linux Mint 20.2 (uma).

Is it a bug or am I doing something wrong?

Start with the static-linking instructions and the pyoxidizer.bzl configuration, then inspect the setup.py installation of greenlet where the musl-clang command fails. Reproduce with pyoxidizer build --target-triple x86_64-unknown-linux-musl and verify whether the required toolchain is discoverable on Linux Mint. Done means the static build completes without the missing-command error.

Written by the indexing model from the issue text.