Add a CI job to test building with unvendored harfbuzz USE_SYSTEM_LIBS=1

[anthrotype]

https://github.com/harfbuzz/uharfbuzz/pull/201 added support for optionally linking uharfbuzz with a preinstalled harfbuzz library, this works but is not currently tested on the CI. To ensure this continues to work we should add a job to test building with USE_SYSTEM_LIBS=1, at least on Linux where this is most likely to be used.