[6.0] Update install to ensure the build directory is not added to the rpath

[grynspan]

Per @bnbarham:

This seems to be fine with the existing CMake, but in 3.29 (possibly earlier) the rpath of libTesting still has the build directory. Make sure to remove any existing build rpath.

• Explanation: Cherry-picks changes to ensure the build directory is not added to the rpath.
• Scope: Toolchain build.
• Issues: N/A
• Original PRs: Update install to ensure the build directory is not added to the rpath #792
• Risk: Low
• Testing: Local CI run, will presumably need a complete toolchain build to very
• Reviewers: @bnbarham @shahmishal @etcwilde @briancroom @stmontgomery

[grynspan]

@swift-ci test

[etcwilde]

I was just about to cherry-pick this. LGTM.

[grynspan]

The Windows failure is a known issue in SwiftPM on main. @dschaefer2 is looking at it.

[grynspan]

@swift-ci test Windows

[dschaefer2]

You need to make your Testing library product with type .dynamic if you want it to export symbols with the 6.1-dev Windows toolchain.

[grynspan]

It is beyond the scope of this PR to change the package manifest settings.

We can just ignore the Windows failure for now, I suppose, since we know what's causing it.