CI(OSGeo4W): Add -pipe to CFLAGS and CXXFLAGS to reduce build time (O…

…SGeo#4326)

With the current GitHub Hosted runners, the build step time is reduced by about 2 minutes, down to about 8min30 instead of about 10min30, for the couple builds I observed

.github/workflows/osgeo4w.yml

@@ -66,7 +66,10 @@ jobs:
 
       - name: Compile GRASS GIS
         shell: msys2 {0}
-        run: .github/workflows/build_osgeo4w.sh
+        run: |
+          export CFLAGS="${CFLAGS} -pipe"
+          export CXXFLAGS="${CXXFLAGS} -pipe"
+          .github/workflows/build_osgeo4w.sh
 
       - name: Print installed versions
         if: always()