CI: Explicitly build before running tests

If we don't do this, Meson will build everything that is a direct
dependency of the test executables, but it won't build indirect
dependencies like GLib's gspawn-win32-helper.exe.

Signed-off-by: Simon McVittie <smcv@collabora.com>

.gitlab-ci.yml

@@ -362,6 +362,7 @@ windows-meson-vs15-x86:
   script:
     - cmd.exe /C "C:\BuildTools\Common7\Tools\VsDevCmd.bat -host_arch=amd64 -arch=x86 &&
         meson --force-fallback-for=expat,glib,zlib build &&
+        meson compile -C build &&
         meson test -C build"
   artifacts:
     reports: