docs/releasing: build test the scons/mingw build
docs/releasing: build test the scons/mingw build

We had multiple cases in the past where files used only by the
Scons/MinGW/Windows build were missing.

Avoid such instances and add a step to catch them early.

Signed-off-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>
Reviewed-by: Eric Engestrom <de0b63cf2dfb1fae8ed3c3b4244856a05677d4f0@imgtec.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler