[CMake] Tweak path separators in new Swift runpaths

[finagolfin]

@rintaro and @bnbarham, this is a quick tweak pull to fix the double slashes in the runpaths shown here after #68082:

> readelf -d usr/lib/swift/host/plugins/lib*so | ag "File:|runpath"
File: usr/lib/swift/host/plugins/libObservationMacros.so
 0x000000000000001d (RUNPATH)            Library runpath: [$ORIGIN/../../..//swift/linux:$ORIGIN/..]
File: usr/lib/swift/host/plugins/libSwiftMacros.so
 0x000000000000001d (RUNPATH)            Library runpath: [$ORIGIN/../../..//swift/linux:$ORIGIN/..]

[rintaro]

Thank you! To clarify, this doesn't affect the actual .so path resolution, right? (i.e. we don't need this for 5.9)

[rintaro]

@swift-ci Please smoke test

[finagolfin]

this doesn't affect the actual .so path resolution, right?

Yes, this should have no functional effect, purely for looks.

[finagolfin]

Linux CI broke because of my lldb rpath pull that was just merged, I'll update the failing integration test in a couple minutes.

[finagolfin]

OK, hopefully linux CI will pass now.

[bnbarham]

@swift-ci please smoke test Linux platform