workflows: Do a build on Steam Runtime 2 'soldier'

[smcv]

This is one of the older environments that we aim to support, so building on here will avoid newer-than-intended dependencies like #14106.

[smcv]

To do:

• try reverting the fixes for [3.3.x] Wayland driver requires newer libxkbcommon than is available in Steam Runtime 2 'soldier' #14101 and make sure the build fails
• maybe we need to make the builds in older environments use -DSDL_DEPS_SHARED=OFF to make them more fragile and more likely to fail the build, rather than successfully building non-functional binaries
• might need -DSDL_KMSDRM=OFF because the kmsdrm libraries in soldier are too old (but for the Steam Runtime use-case, kmsdrm is uninteresting anyway)
• use -Wno-incompatible-pointer-types for the PulseAudio driver if necessary