.github: update apt before installing, in case it is stale

Symptom: assert-test-linux failed in https://github.com/tinygo-org/tinygo/runs/4721983373
with following errors:

E: Failed to fetch http://azure.archive.ubuntu.com/ubuntu/pool/main/c/ceph/librados2_15.2.14-0ubuntu0.20.04.1_amd64.deb  404  Not Found [IP: 52.154.174.208 80]
E: Failed to fetch http://azure.archive.ubuntu.com/ubuntu/pool/main/c/ceph/librbd1_15.2.14-0ubuntu0.20.04.1_amd64.deb  404  Not Found [IP: 52.154.174.208 80]
E: Failed to fetch http://azure.archive.ubuntu.com/ubuntu/pool/main/q/qemu/qemu-block-extra_4.2-3ubuntu6.18_amd64.deb  404  Not Found [IP: 52.154.174.208 80]
E: Failed to fetch http://azure.archive.ubuntu.com/ubuntu/pool/main/q/qemu/qemu-system-common_4.2-3ubuntu6.18_amd64.deb  404  Not Found [IP: 52.154.174.208 80]
E: Failed to fetch http://azure.archive.ubuntu.com/ubuntu/pool/main/q/qemu/qemu-system-data_4.2-3ubuntu6.18_all.deb  404  Not Found [IP: 52.154.174.208 80]
E: Failed to fetch http://azure.archive.ubuntu.com/ubuntu/pool/main/q/qemu/qemu-system-arm_4.2-3ubuntu6.18_amd64.deb  404  Not Found [IP: 52.154.174.208 80]
E: Failed to fetch http://azure.archive.ubuntu.com/ubuntu/pool/main/q/qemu/qemu-system-misc_4.2-3ubuntu6.18_amd64.deb  404  Not Found [IP: 52.154.174.208 80]
E: Failed to fetch http://azure.archive.ubuntu.com/ubuntu/pool/universe/q/qemu/qemu-user_4.2-3ubuntu6.18_amd64.deb  404  Not Found [IP: 52.154.174.208 80]
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?

This usually means those packages have been updated and you have to run apt-get update first, as recommended at
https://github.com/actions/virtual-environments/issues/1757

.github/workflows/linux.yml

@@ -140,6 +140,7 @@ jobs:
           submodules: true
       - name: Install apt dependencies
         run: |
+          sudo apt-get update
           sudo apt-get install --no-install-recommends \
               qemu-system-arm \
               qemu-system-riscv32 \