feat: include mutagen binary in installer

[deansheather]

Publish.ps1 now downloads (if unmodified) the necessary mutagen files from our GCS bucket. These files aren't checked into the repo since they are large, but they will be cached and ignored in the workdir.

mutagen.exe and mutagen-agents.tar.gz will appear in the same directory as wintun.dll in the install.

PS C:\Users\dean\git\coder-desktop-windows> ls .\publish\buildtemp-1.2.3.4-x64\vpn


    Directory: C:\Users\dean\git\coder-desktop-windows\publish\buildtemp-1.2.3.4-x64\vpn


Mode                 LastWriteTime         Length Name
----                 -------------         ------ ----
-a----         1/03/2025     12:02           5431 LICENSE.WINTUN.txt
-a----        10/03/2025     11:58       17964207 mutagen-agents.tar.gz
-a----        10/03/2025     11:58       13797376 mutagen.exe
-a----         1/03/2025     15:20         427552 wintun.dll

Updating to a new mutagen version is as simple as updating $mutagenVersion in Publish.ps1.

Closes #25