Limit enabling zstd tests for nightly rustc to Linux (where rust-lld is shipped).

Author: khuey

.github/workflows/main.yml

@@ -93,7 +93,8 @@ jobs:
         env:
           RUSTFLAGS: "-C link-arg=-Wl,--compress-debug-sections=zlib"
       - run: cargo test
-        if: contains(matrix.os, 'ubuntu-24.04') || contains(matrix.rust, 'nightly')
+        if: contains(matrix.os, 'ubuntu-24.04') ||
+	    (contains(matrix.os, 'ubuntu') && contains(matrix.rust, 'nightly'))
         env:
           RUSTFLAGS: "-C link-arg=-Wl,--compress-debug-sections=zstd"