tests: rename checks in github actions

I usually want to look at the Travis CI output, but I currently have to
scroll down to see it. This renames checks to be a bit shorter and more
consistent with Travis's naming, and also so that actions appear lower
than travis and codecov in the list of checks.

README.md

@@ -1,7 +1,7 @@
 # <img src="https://cdn.rawgit.com/spack/spack/develop/share/spack/logo/spack-logo.svg" width="64" valign="middle" alt="Spack"/> Spack
 
 [![Build Status](https://travis-ci.org/spack/spack.svg?branch=develop)](https://travis-ci.org/spack/spack)
-[![](https://github.com/spack/spack/workflows/Build%20Tests%20Linux/badge.svg)](https://github.com/spack/spack/actions)
+[![Linux Builds](https://github.com/spack/spack/workflows/linux%20builds/badge.svg)](https://github.com/spack/spack/actions)
 [![codecov](https://codecov.io/gh/spack/spack/branch/develop/graph/badge.svg)](https://codecov.io/gh/spack/spack)
 [![Read the Docs](https://readthedocs.org/projects/spack/badge/?version=latest)](https://spack.readthedocs.io)
 [![Slack](https://spackpm.herokuapp.com/badge.svg)](https://spackpm.herokuapp.com)