[bootstrapper] Improve handling of invalid hash

Description

I receive a

The file 'C:\Windows\system32\config\systemprofile\AppData\Local\NuGet\Cache\Paket\5.92.3\paket.exe' already exists. 

error when calling "paket.exe restore" having bumped the version to 5.92.3.

Have never seen this before. It goes away on a brand new build agent (they're automated), but comes back again.

Repro steps

Not yet able to reproduce. Known aspects:

• Occurs when calling paket.exe restore
• Most branches on build server running on 5.85.1 with paket.exe in magic mode, with version passed as argument.
• New branch on build server running 5.92.3 with paket.exe in magic mode, with version passed in paket.dependencies.
• Build servers rebuilt from scratch each day, and happened two days running so far.
• Works successfully on first run of the day.
• Subsequent builds of branch fail on that build server. Not sure if there is interaction with other builds going on here.
• Happened once on local machine but haven't been able to reproduce since after clearing cache folder.

Expected behavior

paket.exe restores packages as in previous versions.

Actual behavior

The file 'C:\Windows\system32\config\systemprofile\AppData\Local\NuGet\Cache\Paket\5.92.3\paket.exe' already exists.

Packages are then not restored.

Known workarounds

Kill the build server and force a fresh instance.

Start with the bootstrapper path used by paket.exe restore and investigate how it handles an existing cached executable at the reported Paket version. Reproduce the failure with repeated builds or cache reuse if possible; done means restore succeeds without requiring the build server to be rebuilt.