automation: set PATH when building on Windows
Sometime in the 6.2 release cycle the Windows building automation
broke. Building the wheel and even PyOxidizer based installers now fails
with:
```
Exception: PowerShell execution failed: error: subprocess-exited-with-error
Getting requirements to build wheel did not run successfully.
exit code: 1
[1 lines of output]
Unable to find a working hg binary to extract the version from the repository tags
[end of output]
note: This error originates from a subprocess, and is likely not a problem with pip.
```
I have a hunch this is a regression from upgrading pip in 1c00777702da,
but I haven't verified this. It may not be, as PyOxidizer has its own
bundled Python/pip. So maybe it is something in `setup.py`.
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
for software developers.
Basic install::
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
$ ./hg --version # should show the latest version
See https://mercurial-scm.org/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.
Notes for packagers
===================
Mercurial ships a copy of the python-zstandard sources. This is used to
provide support for zstd compression and decompression functionality. The
module is not intended to be replaced by the plain python-zstandard nor
is it intended to use a system zstd library. Patches can result in hard
to diagnose errors and are explicitly discouraged as unsupported
configuration.