When adding a local mirror it does not get a higher priority than the public mirror. That causes spack to go out to the Internet too frequently. The only way that I can get it to be a higher priority is to add it to the defaults scope but that does not seem like the right thing to do.

Steps to reproduce the issue

$ spack mirror list
spack-public    https://spack-llnl-mirror.s3-us-west-2.amazonaws.com/
$ spack mirror add test file:///opt/mirror
$ spack mirror list
spack-public    https://spack-llnl-mirror.s3-us-west-2.amazonaws.com/
test            file:///opt/mirror

Downloads will check the public mirror first.

Information on your system

• Spack: 0.11.2-8577-8ad581e
• Python: 2.7.5
• Platform: linux-centos7-skylake_avx512

Additional information

• I have run spack debug report and reported the version of Spack/Python/Platform
• I have searched the issues of this repo and believe this is not a duplicate
• I have run the failing commands in debug mode and reported the output