Fix eBPF target availability issue

- Use nightly-2024-02-15 toolchain which supports bpfel-unknown-none
- Resolves 'rust-std unavailable for bpfel-unknown-none' error
- This version is known to have stable eBPF target support

Author: AndreaBozzo

.github/workflows/core-build-checks.yml

@@ -20,7 +20,7 @@ jobs:
     - name: Install Rust toolchain
       uses: dtolnay/rust-toolchain@master
       with:
-        toolchain: nightly
+        toolchain: nightly-2024-02-15
         targets: bpfel-unknown-none
         components: rustfmt, clippy