CI: build: fix ccache cache usage

CCache cache is currently broken due to a funny bug in ccache compiler
type detection. It seems ccache compiler type detection is very fragile
and with the use of external toolchain doesn't correctly detect the
type.
The type detected is set to other instead of gcc resulting in ccache
complaining for unsupported compiler options.

To handle this problem, force the compiler type to gcc to make ccache
correctly work and speedup compilation.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>

.github/workflows/build.yml

@@ -302,11 +302,22 @@ jobs:
           echo CONFIG_TARGET_PER_DEVICE_ROOTFS=y >> .config
           echo CONFIG_TARGET_ALL_PROFILES=y >> .config
 
-      - name: Configure ccache
+        # ccache for some reason have problem detecting compiler type
+        # with external toolchain. This cause the complete malfunction
+        # of ccache with the result of tons of unsupported compiler
+        # option error.
+        # To fix this force compiler type to gcc.
+      - name: Configure ccache and apply fixes
         if: inputs.use_ccache_cache == true
         shell: su buildbot -c "sh -e {0}"
         working-directory: openwrt
+        env:
+          SYSTEM_CCACHE_CONF: staging_dir/host/etc/ccache.conf
         run: |
+          touch $SYSTEM_CCACHE_CONF
+
+          echo compiler_type=gcc >> $SYSTEM_CCACHE_CONF
+
           echo CONFIG_CCACHE=y >> .config
 
       - name: Configure external toolchain in container