ci: fix intermittent fontconfig SIGSEGV on Linux CI (#309451)

The Electron main process intermittently crashes during startup on the
`[pango] FcInit` thread with a NULL pointer dereference in expat's XML
string processing, triggered by fontconfig parsing `<include>` directives
in fonts.conf via `XML_ExternalEntityParserCreate`.

Set FONTCONFIG_FILE to a minimal config based on upstream
fontconfig 2.15.0 fonts.conf.in with `<include>` directives removed and
generic family aliases inlined. This avoids the external entity parser
codepath entirely. A version check will fail the build once the runner
ships expat >= 2.7.5, prompting removal of the workaround.