github: Fix build on Ubuntu 20.04 and add a job for 18.04

The nm test can only be done on Ubuntu 18.04 as the required libraries
are not available on newer systems.

Switch to pip3 to install tox (the only Python dependency we use).

Closes strongswan/strongswan#327.

.github/workflows/linux.yml

@@ -44,7 +44,6 @@ jobs:
           - test: apidoc
           - test: coverage
           - test: dist
-          - test: nm
           - test: nm-no-glib
           - test: fuzzing
             compiler: clang
@@ -121,6 +120,44 @@ jobs:
           path: config.log
           retention-days: 5
 
+  bionic:
+    needs: pre-check
+    if: ${{ needs.pre-check.outputs.should_skip != 'true' }}
+    runs-on: ubuntu-18.04
+    strategy:
+      matrix:
+        test: [ all ]
+        compiler: [ gcc, clang ]
+        include:
+          - test: nm
+    env:
+      LEAK_DETECTIVE: ${{ matrix.leak-detective || 'no' }}
+      CC: ${{ matrix.compiler || 'gcc' }}
+      TEST: ${{ matrix.test }}
+      UBUNTU_BIONIC: yes
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/cache@v2
+        with:
+          path: ~/.ccache
+          key: ccache-bionic-${{ env.CC }}-${{ matrix.test }}-${{ github.ref }}:${{ github.sha }}
+          restore-keys: |
+            ccache-bionic-${{ env.CC }}-${{ matrix.test }}-${{ github.ref }}:
+            ccache-bionic-${{ env.CC }}-${{ matrix.test }}-
+            ccache-bionic-${{ env.CC }}-
+      - run: |
+          sudo apt-get install -qq ccache
+          echo "PATH=/usr/lib/ccache:$PATH" >> $GITHUB_ENV
+          ccache -z
+      - uses: ./.github/actions/default
+      - run: ccache -s
+      - if: ${{ failure() }}
+        uses: actions/upload-artifact@v2
+        with:
+          name: Logs ${{ github.job }}
+          path: config.log
+          retention-days: 5
+
   xenial:
     needs: pre-check
     if: ${{ needs.pre-check.outputs.should_skip != 'true' }}
@@ -137,7 +174,6 @@ jobs:
       LEAK_DETECTIVE: ${{ matrix.leak-detective || 'no' }}
       CC: ${{ matrix.compiler || 'gcc' }}
       TEST: ${{ matrix.test }}
-      UBUNTU_XENIAL: yes
       # this is the default with newer versions and fixes builds with clang
       CCACHE_CPP2: true
     steps: