ci: Select number of parallel make jobs depending on CI environment

This should improve compilation times on macOS. Things can certainly
be improved further, e.g., by running the benchmarks in parallel.

ci/cirrus.sh

@@ -21,7 +21,8 @@ valgrind --version || true
     --with-valgrind="$WITH_VALGRIND" \
     --host="$HOST" $EXTRAFLAGS
 
-make -j2
+# We have set "-j<n>" in MAKEFLAGS.
+make
 
 # Print information about binaries so that we can see that the architecture is correct
 file *tests || true
@@ -30,7 +31,7 @@ file .libs/* || true
 
 if [ -n "$BUILD" ]
 then
-    make -j2 "$BUILD"
+    make "$BUILD"
 fi
 
 if [ "$RUN_VALGRIND" = "yes" ]