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.

.github/workflows/build_tests_linux.yaml → .github/workflows/linux_build_tests.yaml

-name: Build Tests Linux
+name: linux builds
 
 on:
   push:

.github/workflows/minimum_python_versions.yaml

-name: Minimum Python Versions
+name: python version check
 
 on:
   push:
@@ -10,7 +10,7 @@ on:
       - master
       - develop
 jobs:
-  build:
+  validate:
 
     runs-on: ubuntu-latest
 
@@ -19,7 +19,7 @@ jobs:
     - name: Setup Python
       uses: actions/setup-python@v1
       with:
-        python-version: 2.7
+        python-version: 3.7
     - name: Install Python Packages
       run: |
         pip install --upgrade pip

README.md

 # <img src="https://cdn.rawgit.com/spack/spack/develop/share/spack/logo/spack-logo.svg" width="64" valign="middle" alt="Spack"/> Spack
 
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+[![Linux Builds](https://github.com/spack/spack/workflows/linux%20builds/badge.svg)](https://github.com/spack/spack/actions)
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