GHA/linux: fix 'mbedtls' internal name confusion

GHA/linux: fix 'mbedtls' internal name confusion

`mbedtls-arm` and `mbedtls-prev` were matching `contains()` expressions
looking for `mbedtls`. It caused an unnecessary cache restore and an
redundant mbedtls build on bumps, and made the build flavors require
different local directory names to avoid this accidental collision.

Also drop `-threadsafe` from internal names. All local builds are.

Follow-up to 88060353440df4e1b7167c180b39074fd2ab457a #20240
Follow-up to 3a305831d1a9d10b2bfd4fa3939ed41275fee7f7 #19077

Closes #20248