Support custom build commands for Swift static Linux/Wasm SDK builds

[jrflat]

Enable alias expansion and run alias swift="$SWIFT_EXECUTABLE_FOR_(NAME)_SDK" before performing the build so that we can support custom swift build commands while still using the correct executable.

I'll add a commit that points to the script URL on my branch for testing this PR, then remove it before merging.

Resolves #147

[shahmishal]

Looks good, make sure to review the forked repo path once test is completed.

• Revert the script back to swiftlang/github-workflows url

[jrflat]

Reverted the script URL back to swiftlang:main. Previous successful run for reference: https://github.com/swiftlang/github-workflows/actions/runs/16738403255

[jrflat]

This is going to conflict with the name/description changes in #150. I'll wait for that to land or remove my changes that'll conflict.

Update: Removed the description changes that would conflict from this PR.

[jrflat]

I'm going to squash commits and re-test to make the PR branch testing dance clearer

[jrflat]

Previous commit using this branch for the script URL is green. I've now reverted the script URL back to swiftlang:main. This PR also no longer conflicts with Max's PR.

@shahmishal I think this is ready to merge if it looks good to you, thanks!

[jakepetroules]

Thanks for landing this so quickly!