Support arbitrary toolchains with cargo wrapper script (#12926)

(cherry picked from commit 99aecdaf65)

Co-authored-by: Trent Nelson <trent@solana.com>

cargo

@@ -17,6 +17,10 @@ case "$1" in
     toolchain="$rust_nightly"
     shift
     ;;
+  +*)
+    toolchain="${1#+}"
+    shift
+    ;;
   *)
     # shellcheck disable=SC2054 # rust_stable is sourced from rust-version.sh
     toolchain="$rust_stable"