Reduce ccache size for gcc to 3G

To avoid having jobs being unable to generate the cache like
https://gitlab.com/OpenMW/openmw/-/jobs/5766643208:

Creating cache Ubuntu_GCC.ubuntu_22.04.v1-3-non_protected...
apt-cache/: found 1082 matching artifact files and directories
ccache/: found 7443 matching artifact files and directories
FATAL: write ../../../cache/OpenMW/openmw/Ubuntu_GCC.ubuntu_22.04.v1-3-non_protected/archive_991882895: no space left on device
Failed to create cache

.gitlab-ci.yml

@@ -142,7 +142,7 @@ Ubuntu_GCC:
   variables:
     CC: gcc
     CXX: g++
-    CCACHE_SIZE: 4G
+    CCACHE_SIZE: 3G
   # When CCache doesn't exist (e.g. first build on a fork), build takes more than 1h, which is the default for forks.
   timeout: 2h
 
@@ -193,7 +193,7 @@ Ubuntu_GCC_Debug:
   variables:
     CC: gcc
     CXX: g++
-    CCACHE_SIZE: 4G
+    CCACHE_SIZE: 3G
     CMAKE_BUILD_TYPE: Debug
     CMAKE_CXX_FLAGS_DEBUG: -O0
   # When CCache doesn't exist (e.g. first build on a fork), build takes more than 1h, which is the default for forks.