ruby: limit -j in build phase

High -j results in errors in fixup_mmaps_after_fork apparently due to
insufficient page file space.

See https://cygwin.com/pipermail/cygwin/2025-January/257155.html

ruby/PKGBUILD

@@ -93,7 +93,8 @@ build() {
     --with-dbm-type=gdbm_compat \
     LDSHARED="gcc -shared" \
     DLDFLAGS="-Wl,--export-all-symbols"
-  make
+  # -j2 due to excessive paging file usage
+  make -j2
 }
 
 check() {