Bump ubuntu version and use fetched libgccjit

Don't use system's libgccjit + bump ubuntu to have recent enough glibc

Refs: compiler-explorer/compiler-explorer#8147

Author: dkm

Dockerfile.rust-cg-gcc

@@ -1,4 +1,4 @@
-FROM ubuntu:22.04
+FROM ubuntu:24.04
 
 ARG DEBIAN_FRONTEND=noninteractive
 RUN dpkg --add-architecture i386

rust-cg-gcc/build.sh

@@ -76,6 +76,18 @@ pushd rustc_codegen_gcc
 cp config.example.toml config.toml
 
 ./y.sh prepare
+
+# This may fail because it can't find libgccjit that it downloads itself....
+./y.sh build --sysroot --release || true
+
+# So restart it after the failed execution.
+# Following y.sh needs to find the libgccjit that is getting downloaded
+SO_PATH=$(find -name "libgccjit.so")
+LIBRARY_PATH=$(dirname "$PWD"/"$SO_PATH")
+LD_LIBRARY_PATH=$(dirname "$PWD"/"$SO_PATH")
+export LIBRARY_PATH
+
+# ... this one should work.
 ./y.sh build --sysroot --release
 
 popd