ci/android: unset compiler env vars in debian/android_build.sh

Unset the compiler env vars in debian/android_build.sh used to
cross-build libelf for Android, to prevents those env vars to
inadvertently affect subsequent commands that might want to build for
the native host after the cross-builds.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31093>

.gitlab-ci/container/debian/android_build.sh

@@ -89,6 +89,12 @@ for arch in \
                 --libdir=/usr/local/lib/${arch}
     make install
     make distclean
+
+    unset CC
+    unset CC
+    unset CXX
+    unset LD
+    unset RANLIB
 done
 
 cd ..