Pin to Ubuntu 18.04, because the new 20.04 results in this error on my

18.04 system:

/lib/x86_64-linux-gnu/libm.so.6: version `GLIBC_2.29' not found

I'm probably not the only one on an older Ubuntu, and from the last time
I went through this dance, I remember newer systems being able to handle
binaries built on older, but not vice versa.

actionsvirtual-environments#1816

.github/workflows/main.yml

@@ -14,7 +14,7 @@ jobs:
         build: [linux, macos, windows]
         include:
           - build: linux
-            os: ubuntu-latest
+            os: ubuntu-18.04
           - build: macos
             os: macos-latest
           - build: windows
@@ -39,7 +39,7 @@ jobs:
         run: if [[ `git log -1 --pretty=format:"%s"` == *"[release]"* ]]; then echo 'features=--features=map_gui/release_s3' >> $GITHUB_ENV; else echo 'features=' >> $GITHUB_ENV; fi
 
       - name: Install dependencies
-        if: matrix.os == 'ubuntu-latest'
+        if: matrix.os == 'ubuntu-18.04'
         run: sudo apt-get install libasound2-dev
 
       - name: Build game
@@ -71,7 +71,7 @@ jobs:
         shell: bash
 
       - name: Upload release for Linux
-        if: matrix.os == 'ubuntu-latest'
+        if: matrix.os == 'ubuntu-18.04'
         uses: actions/upload-artifact@v2
         with:
           name: abst_linux

release/build.sh

@@ -5,7 +5,7 @@ set -e;
 
 os=$1;
 case $os in
-	ubuntu-latest)
+	ubuntu-18.04)
 		output="abst_linux";
 		suffix="";
 		runner="play_abstreet.sh";