fips: Support LIBGCRYPT_FORCE_FIPS_MODE envvar.

* src/fips.c (_gcry_initialize_fips_mode): Check the
environment variable LIBGCRYPT_FORCE_FIPS_MODE.

Useful for developers to test FIPS mode.

• GnuPG-bug-id: T5541
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>