Run the Makefile's SwiftPM bin path lookup once per make process

[deadcaf3]

Type of Change

• Bug fix

Motivation and Context

Fixes #2346.

BUILD_BIN_DIR was a recursive variable around $(shell swift build --show-bin-path), so every reference started SwiftPM again: 20 times for make all, about 12 s. It is now defined with :=, so each make process looks the path up once.

This is the line #221 changed from := to = because BUILD_CONFIGURATION changes during make release. Going back is safe for the current Makefile: the recipes that use the path under release run in the sub-make, which is started with BUILD_CONFIGURATION=release and resolves the release path when it parses the Makefile. STAGING_DIR, PKG_PATH and DSYM_DIR already rely on that.

One difference from =: a recipe that reads $(BUILD_BIN_DIR) in the top-level make under release would get the debug path. No target does that today, and none did when #221 landed: make -n release uses the release path both at #221 and at its parent commit.

Testing

• Tested locally

• make -n output for all, release, cli, dsym and coverage is byte-identical before and after. All 8 bin paths in make -n release stay under .build/out/Products/Release.

• SwiftPM lookups in make -n all go from 20 to 2 (one per make process); coverage from 32 to 3; fmt and clean from 6 to 1.

• time make -n all: 12.15 s, 11.98 s and 11.46 s before; 1.30 s, 1.37 s and 1.26 s after.

• make all and make test pass.

GNU Make 3.81, macOS 26.6.2, Xcode 27.0.