actions: pin Ubuntu version

ubuntu-latest is now assigned to Ubuntu 20.04. As we use custom apt
sources for 18.04, pin to this version for now to fix the CI.

.github/workflows/build-gluon.yml

@@ -33,7 +33,7 @@ jobs:
       fail-fast: false
       matrix:
         target: [ar71xx-generic, ar71xx-tiny, ar71xx-nand, ath79-generic, brcm2708-bcm2708, brcm2708-bcm2709, ipq40xx-generic, ipq806x-generic, lantiq-xrx200, lantiq-xway, mpc85xx-generic, mpc85xx-p1020, ramips-mt7620, ramips-mt7621, ramips-mt76x8, ramips-rt305x, sunxi-cortexa7, x86-generic, x86-geode, x86-legacy, x86-64, ar71xx-mikrotik, brcm2708-bcm2710, mvebu-cortexa9]
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
       - uses: actions/checkout@v1
       - name: Install Dependencies

contrib/actions/generate-actions.py

@@ -37,7 +37,7 @@ jobs:
       fail-fast: false
       matrix:
         target: [{matrix}]
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
       - uses: actions/checkout@v1
       - name: Install Dependencies