Add an opt-in check for only linking FoundationEssentails on Linux

[czechboy0]

In some Swift Server libraries, we're trying to minimize which individual Foundation sub-modules we link, and it'd be useful to have a CI check to avoid a PR accidentally regressing it.

Roughly could work like this:

• a script temporarily adds an extra executable product to the package that imports the library in the package that we want to test for not linking full Foundation on Linux
• swift build that product
• run ldd on that product in the .build directory
• ensure that libFoundation.so is not present in there, otherwise fail the CI job