Strip trailing slash from source dir for cmake4

[am11]

Alternative fix for #114766.

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[akoeplinger]

why was the existing code to remove the trailing slash not working?

[am11]

It was not self-assigning __sourceDir (only __ProjectDir). Now it assigns both.

[akoeplinger]

ah ok got it but I think that might've been a copy/paste mistake, I don't think it should assign __ProjectDir, just strip the trailing slash from __sourceDir

[akoeplinger]

/ba-g failures are not related to the change

[akoeplinger]

/backport to release/9.0-staging

[github-actions]

Started backporting to release/9.0-staging: https://github.com/dotnet/runtime/actions/runs/14593733890

[akoeplinger]

/backport to release/8.0-staging

[github-actions]

Started backporting to release/8.0-staging: https://github.com/dotnet/runtime/actions/runs/14593740104