PyOxidizer inherits head revision of working repo

I tried to give 0.8.0 a go. When PyOxidizer got to the step of building pyembed, it failed with:

```
[...]
Updating git repository `https://github.com/indygreg/PyOxidizer.git`
error: failed to get `pyembed` as a dependency of package `instawow v0.1.0 (/private/var/folders/18/r2nky_v137d3fwhtx0644m940000gn/T/pyoxidizer.c3yteoPgY6er/instawow)`

Caused by:
failed to load source for dependency `pyembed`

Caused by:
Unable to update https://github.com/indygreg/PyOxidizer.git?rev=255cc182f77c995489bba1fa46300ee73cd41af0

Caused by:
revspec '255cc182f77c995489bba1fa46300ee73cd41af0' not found; class=Reference (4); code=NotFound (-3)
error[PYOXIDIZER_BUILD]: cargo build failed
--> ./pyoxidizer.bzl:190:5
|
190 | files.add_python_resource(".", exe)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ add_python_resource
```

`255cc182f77c995489bba1fa46300ee73cd41af0` is not a commit in the PyOx repo; it's the head of my repo and it is the commit value of `PYOXIDIZER_COMMIT` in pyoxidizer.bzl. Changing the latter to

```
PYOXIDIZER_COMMIT = "270a69e084a99907c32be10f315f5a447c7486d9"
```

had no effect. `pyoxidizer --version` also shows the commit from my repo:

```
PyOxidizer 0.8.0
commit: 255cc182f77c995489bba1fa46300ee73cd41af0
source: https://github.com/indygreg/PyOxidizer.git
pyembed crate location: git = "https://github.com/indygreg/PyOxidizer.git", rev = "255cc182f77c995489bba1fa46300ee73cd41af0"
```

Start by reproducing the 0.8.0 build failure and inspect pyoxidizer.bzl around line 190, including PYOXIDIZER_COMMIT. Check how the build resolves the pyembed dependency and how `pyoxidizer --version` obtains its commit value. Done means the release uses a valid PyOxidizer revision rather than the reporter's working-repository head.

Written by the indexing model from the issue text.