Skip internal signing phase unless on windows/mac

[sxa]

The implementation of the internal signing phase which was adjusted in #1117 (issue) caused that phase to be executed on more platforms than is necessary.

There are two fixes in here:

• trim() the output from ls when setting build_path so that it doesn't have a newline character on it (This was already done for windows but not on mac, but is required there too).
• Ensure that the internal signing + assemble phases are only executed on macos and windows. It was trying to run on Linux platforms and tripping up on the stash operations

Notes / alternative options:

• We could have a separate build flag for the internal signing instead of using ENABLE_SIGNER for both signing situations instead of doing this on a platform basis (probably a good idea in the future)
• This has been tested on Linux and macos and appears to do what is expected. Windows will take a little longer due to build times, but since macos looks ok this is likely safe to merge now.
• signing issues are not caught during the PR testing process. We could perhaps look at incorporating that in the future.

[github-actions]

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[steelhead31]

Looks good.