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

[anthrotype]

#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.