test-msys2-meson: fix pipeline failure on GTK3
On GTK3 the msys2-mingw64-meson job is failing due to error: mingw-w64-x86_64-pkg-config and mingw-w64-x86_64-pkgconf are in conflict. Remove mingw-w64-x86_64-pkgconf? [Y/n]
To fix this we change required package pkg-config to pkgconf, which is what the GTK4 branch also uses.
Edited by Jacob Boerema