workflows: build-polars-runtime: change Cargo optimizations

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>

Author: threexc

.github/workflows/build-polars-runtime.yml

@@ -30,6 +30,19 @@ env:
   CARGO_INCREMENTAL: 0
   CARGO_NET_RETRY: 10
   RUSTUP_MAX_RETRIES: 10
+  # `--profile dist-release` (see "Build wheel" below) inherits fat LTO +
+  # codegen-units=1 from py-polars' Cargo.toml. Fat LTO is a whole-program
+  # optimization pass whose memory use scales with program size, not
+  # parallelism - it's the likely cause of exhausting a resource-constrained
+  # riscv64 runner regardless of job count. Overriding it here keeps the
+  # dist-release profile name/other settings intact without patching
+  # Cargo.toml.
+  CARGO_PROFILE_DIST_RELEASE_LTO: thin
+  CARGO_PROFILE_DIST_RELEASE_CODEGEN_UNITS: '16'
+  # Caps concurrent rustc invocations so multiple large crates (polars-plan,
+  # polars-expr, polars-python, ...) don't get monomorphized/codegen'd at
+  # once. This needs to be adjusted if the runners change.
+  CARGO_BUILD_JOBS: '4'
 
 defaults:
   run: