exe-scroll: per-target zig cache dir for reproducible cross builds

Prompt: is the zig build for exe-scroll reproducible? / Can we make it so
that we use a different cache directory

Zig's local .zig-cache isn't fully partitioned by -Dtarget, so building
both arches back-to-back through one checkout produced a non-reproducible
second binary (its .text differed ~48 bytes from a clean build).

Point 'zig build' at a per-target cache dir (.zig-cache/<triple>, still
gitignored, overridable via CACHE_DIR). Each arch now builds byte-identical
to its clean build regardless of order. Verified amd64->arm64 and
arm64->amd64 both reproduce the clean-build hashes.

Co-authored-by: Shelley <shelley@exe.dev>

Author: philz

exe-scroll/build-static.sh

@@ -77,10 +77,19 @@ ln -sfn "$GHOSTTY_SRC" "$SRC_DIR/ghostty-src"
 # rovol bin tree; defaults to the usual zig-out. -Dstrip strips at link time so
 # cross-arch builds don't depend on a matching host `strip`.
 OUT_DIR="${OUT_DIR:-$SRC_DIR/zig-out}"
+
+# Key the local build cache by target triple. Zig's local `.zig-cache` isn't
+# fully partitioned by `-Dtarget`, so building both arches through one cache
+# yields a non-reproducible second binary (its .text differs from a clean
+# build). A per-target cache dir keeps each arch's build deterministic even
+# when both run back-to-back in the same checkout. Defaults under .zig-cache
+# (gitignored); override with CACHE_DIR.
+CACHE_DIR="${CACHE_DIR:-$SRC_DIR/.zig-cache/$ZIG_TARGET}"
 (
     cd "$SRC_DIR"
     mise exec -- zig build \
         -Dtarget="$ZIG_TARGET" -Doptimize=ReleaseFast -Dstrip=true \
+        --cache-dir "$CACHE_DIR" \
         -p "$OUT_DIR"
 )