github: move build job to Ubuntu 20.04/22.04

Ubuntu 18.04 is deprecated and will be fully removed from GitHub
Actions soon (https://github.com/actions/runner-images/issues/6002)

Signed-off-by: Florian Hofhammer <florian.hofhammer@fhofhammer.de>

.github/workflows/build.yaml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os:     [ubuntu-18.04, ubuntu-20.04]
+        os:     [ubuntu-20.04, ubuntu-22.04]
         mode:   [newlib, linux, musl]
         target: [rv32gc-ilp32d, rv64gc-lp64d]
         exclude:

.github/workflows/nightly-release.yaml

@@ -48,7 +48,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os:     [ubuntu-18.04, ubuntu-20.04]
+        os:     [ubuntu-20.04, ubuntu-22.04]
         mode:   [newlib, linux, musl]
         target: [rv32gc-ilp32d, rv64gc-lp64d]
         exclude: