Generate seperate release names for LLVM and GCC toolchain release artifacts

[patrick-rivos]

In #1264, I changed the name of the generated artifacts in build.yaml, but missed the corresponding name in nightly-release.yaml.
This PR fixes that so the LLVM/GCC generated artifacts don't have a naming clash and overwrite eachother in the release flow.

[TommyMurphyTM1234]

Apologies if it's a dumb question but is this PR aimed at addressing the issue that currently leaves "nightly" builds with no llvm tools? For example:

• https://github.com/riscv-collab/riscv-gnu-toolchain/releases/tag/2023.06.09

[patrick-rivos]

Yep! that's what this PR aims to fix.
FWIW, the riscv32-glibc-ubuntu-20.04-nightly-2023.06.09-nightly.tar.gz and riscv64-glibc-ubuntu-20.04-nightly-2023.06.09-nightly.tar.gz appear to have LLVM tools in them. I would guess that's due to the LLVM builds taking longer than the GCC build+testsuite runtime.

[kito-cheng]

Thanks :)