Stop using prebuilt rsync in installer when not on macOS (#3312)

Fixes running the installer on Linux, which we do on CI.

Signed-off-by: Brentley Jones <github@brentleyjones.com>

Author: brentleyjones

xcodeproj/internal/templates/installer.sh

@@ -90,19 +90,24 @@ readonly dest_xcschemes="$dest/xcshareddata/xcschemes"
 
 mkdir -p "$dest_xcschemes"
 
-"$src_rsync" \
-  --archive \
-  --perms \
-  --chmod=u+w,F-x \
-  --delete \
-  "$src_xcschemes" "$dest_xcschemes/"
-
 if [[ $(uname) == "Darwin" ]]; then
   is_macos=1
+  readonly rsync="$src_rsync"
 else
   is_macos=0
+  readonly rsync="$(command -v rsync || true)"
+  [[ -n "$rsync" ]] || fail \
+    "Failed to locate host rsync." \
+    "rules_xcodeproj's bundled rsync is only supported on macOS project generation."
 fi
 
+"$rsync" \
+  --archive \
+  --perms \
+  --chmod=u+w,F-x \
+  --delete \
+  "$src_xcschemes" "$dest_xcschemes/"
+
 # Resolve the copy command (can't use `cp -c` if the files are on different
 # filesystems, or on Linux)
 if [[ $is_macos -eq 1 &&