scripts/build/combine_libs: use $AR rather than ar

Using a custom ar at $AR may be necessary for cross-compilation.

Closes #40369
Signed-off-by: David Goulet <dgoulet@torproject.org>

changes/ticket40369

+  o Minor feature (build, cross-compilation):
+    - Allow a custom ar for cross-compilation purposes. Closes ticket 40369.

scripts/build/combine_libs

@@ -25,7 +25,7 @@ for input in "$@"; do
     dir="$TMPDIR"/$(basename "$input" .a)
     mkdir "$dir"
     cd "$dir">/dev/null
-    ar x "$abs"
+    "${AR:-ar}" x "$abs"
 done
 
 cd "$TMPDIR" >/dev/null