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`.

contrib/automation/hgautomation/windows.py

@@ -45,6 +45,7 b' Write-Output "updated Mercurial working '
 '''.lstrip()
 
 BUILD_INNO_PYTHON3 = r'''
+$Env:PATH = "C:\hgdev\venv-bootstrap\Scripts;$Env:PATH"
 $Env:RUSTUP_HOME = "C:\hgdev\rustup"
 $Env:CARGO_HOME = "C:\hgdev\cargo"
 Set-Location C:\hgdev\src
@@ -56,6 +57,7 b' if ($LASTEXITCODE -ne 0) {{'
 
 
 BUILD_WHEEL = r'''
+$Env:PATH = "C:\hgdev\venv-bootstrap\Scripts;$Env:PATH"
 Set-Location C:\hgdev\src
 C:\hgdev\python{python_version}-{arch}\python.exe -m pip wheel --wheel-dir dist .
 if ($LASTEXITCODE -ne 0) {{
@@ -64,6 +66,7 b' if ($LASTEXITCODE -ne 0) {{'
 '''
 
 BUILD_WIX_PYTHON3 = r'''
+$Env:PATH = "C:\hgdev\venv-bootstrap\Scripts;$Env:PATH"
 $Env:RUSTUP_HOME = "C:\hgdev\rustup"
 $Env:CARGO_HOME = "C:\hgdev\cargo"
 Set-Location C:\hgdev\src