r-rjava: set JAVAH environment variable (#23055)

Set the path to javah via the JAVAH environment variable. If it is
a version of java that does not have javah it will fall back to `javac
-h`. Without specifying this the build could pick up a javah from the
system.

var/spack/repos/builtin/packages/r-rjava/package.py

@@ -30,3 +30,8 @@ class RRjava(RPackage):
     depends_on('libiconv')
     depends_on('pcre2')
     depends_on('xz')
+
+    def setup_build_environment(self, env):
+        spec = self.spec
+        env.append_flags('JAVAH', '{0}/javah'.format(
+            join_path(spec['java'].prefix.bin)))