Only set env vars if they are unset.

action.yml

@@ -57,15 +57,27 @@ runs:
     # The environment variables always need to be set before the caching action
     - name: "Setting Environment Variables"
       run: |
-        echo "CARGO_INCREMENTAL=0" >> $GITHUB_ENV
-        echo "CARGO_PROFILE_DEV_DEBUG=0" >> $GITHUB_ENV
-        echo "CARGO_TERM_COLOR=always" >> $GITHUB_ENV
-        echo "RUST_BACKTRACE=short" >> $GITHUB_ENV
-        echo "RUSTFLAGS=-D warnings" >> $GITHUB_ENV
+        if [[ ! -v CARGO_INCREMENTAL ]]; then
+          echo "CARGO_INCREMENTAL=0" >> $GITHUB_ENV
+        fi
+        if [[ ! -v CARGO_PROFILE_DEV_DEBUG ]]; then
+          echo "CARGO_PROFILE_DEV_DEBUG=0" >> $GITHUB_ENV
+        fi
+        if [[ ! -v CARGO_TERM_COLOR ]]; then
+          echo "CARGO_TERM_COLOR=always" >> $GITHUB_ENV
+        fi
+        if [[ ! -v RUST_BACKTRACE ]]; then
+          echo "RUST_BACKTRACE=short" >> $GITHUB_ENV
+        fi
+        if [[ ! -v RUSTFLAGS ]]; then
+          echo "RUSTFLAGS=-D warnings" >> $GITHUB_ENV
+        fi
         # Enable faster sparse index on nightly
         # The value is ignored on stable and causes no problems
         # https://internals.rust-lang.org/t/call-for-testing-cargo-sparse-registry/16862
-        echo "CARGO_UNSTABLE_SPARSE_REGISTRY=true" >> $GITHUB_ENV
+        if [[ ! -v CARGO_UNSTABLE_SPARSE_REGISTRY ]]; then
+          echo "CARGO_UNSTABLE_SPARSE_REGISTRY=true" >> $GITHUB_ENV
+        fi
         # Enable sparse index after stabilization
         # This causes warnings on stable 1.67, e.g., when using "cargo add"
         # https://github.com/rust-lang/cargo/pull/11224