Force absolute boot ROM paths in libretro's build bot

libretro/gitlab-ci.yml

@@ -10,7 +10,8 @@
     JNI_PATH: libretro
     MAKEFILE_PATH: libretro
     CORENAME: sameboy
-    BOOTROMS_DIR: BootROMs/prebuilt
+  before_script:
+    - export BOOTROMS_DIR=$(pwd)/BootROMs/prebuilt
 
 # Inclusion templates, required for the build to work
 include:
@@ -178,4 +179,4 @@ libretro-build-libnx-aarch64:
 libretro-build-vita:
   extends:
     - .libretro-vita-static-retroarch-master
-    - .core-defs
+    - .core-defs