fix: compose nightly linker rustflags

Ensure PLATFORM_NIGHTLY_RUSTFLAGS is loaded and RUSTFLAGS composition
preserves existing values while appending optional fast-linker defaults.

Author: echobt

.cargo/config.toml

@@ -24,6 +24,7 @@ jobs = "default"
 # PLATFORM_RUST_NIGHTLY: set to 1 to force nightly flags.
 PLATFORM_DISABLE_NIGHTLY = { value = "${PLATFORM_DISABLE_NIGHTLY}", force = false }
 PLATFORM_RUST_NIGHTLY = { value = "${PLATFORM_RUST_NIGHTLY}", force = false }
+PLATFORM_NIGHTLY_RUSTFLAGS = { value = "${PLATFORM_NIGHTLY_RUSTFLAGS}", force = false }
 # Fast-linker defaults are opt-in: set PLATFORM_FAST_LINKER_RUSTFLAGS
 # or PLATFORM_FAST_LINKER_RUSTFLAGS_DARWIN to enable on a system
 # with the desired linker installed. Set PLATFORM_DISABLE_FAST_LINKER=1
@@ -33,7 +34,7 @@ PLATFORM_FAST_LINKER_RUSTFLAGS = { value = "${PLATFORM_FAST_LINKER_RUSTFLAGS}",
 PLATFORM_FAST_LINKER_RUSTFLAGS_DARWIN = { value = "${PLATFORM_FAST_LINKER_RUSTFLAGS_DARWIN}", force = false }
 PLATFORM_LINKER_RUSTFLAGS = { value = "${PLATFORM_LINKER_RUSTFLAGS}", force = false }
 PLATFORM_LINKER_RUSTFLAGS_DARWIN = { value = "${PLATFORM_LINKER_RUSTFLAGS_DARWIN}", force = false }
-RUSTFLAGS = { value = "${PLATFORM_NIGHTLY_RUSTFLAGS} ${PLATFORM_FAST_LINKER_RUSTFLAGS} ${PLATFORM_LINKER_RUSTFLAGS}", force = false }
+RUSTFLAGS = { value = "${RUSTFLAGS} ${PLATFORM_NIGHTLY_RUSTFLAGS} ${PLATFORM_FAST_LINKER_RUSTFLAGS} ${PLATFORM_LINKER_RUSTFLAGS}", force = true }
 
 [target.'cfg(target_os = "macos")'.env]
-RUSTFLAGS = { value = "${PLATFORM_NIGHTLY_RUSTFLAGS} ${PLATFORM_FAST_LINKER_RUSTFLAGS_DARWIN} ${PLATFORM_LINKER_RUSTFLAGS_DARWIN}", force = false }
+RUSTFLAGS = { value = "${RUSTFLAGS} ${PLATFORM_NIGHTLY_RUSTFLAGS} ${PLATFORM_FAST_LINKER_RUSTFLAGS_DARWIN} ${PLATFORM_LINKER_RUSTFLAGS_DARWIN}", force = true }