flaky CI: File has unexpected size (149055 != 149057). Mirror sync in progress?

[timotheecour]

refs nim-lang#16276
https://github.com/nim-lang/Nim/pull/16276/checks?check_run_id=1520875221

Run sudo apt-fast update -qq
  sudo apt-fast update -qq
  DEBIAN_FRONTEND='noninteractive' \
    sudo apt-fast install --no-install-recommends -yq \
      libcurl4-openssl-dev libgc-dev libsdl1.2-dev libsfml-dev \
      valgrind libc6-dbg libblas-dev xorg-dev
  shell: /bin/bash -e {0}
  env:
    NIM_TEST_PACKAGES: 2
E: Failed to fetch packages.microsoft.com/ubuntu/18.04/prod/dists/bionic/main/binary-amd64/Packages.bz2  File has unexpected size (149055 != 149057). Mirror sync in progress? [IP: 13.93.224.173 443]
   Hashes of expected file:
    - Filesize:149057 [weak]
    - SHA512:51b0d5c928dd8aaaaee1c3eb90ae5f3816d1c4737717a6710d79eb6d263f1ee875788ac1a963cb3aa96ad02266b2298e29fe7b98c5ecb4eee70607c9e01f17f9
    - SHA256:cccda30db5b3f8776ee298d6c5ef1bdfec34cdbf3a8673a6d1a71b892feb0ff6
    - SHA1:9a5931cdcba333db513f9fb39a8c92aa053c9b3d [weak]
    - MD5Sum:f6b322bf712d08dfc80f2e8aa18dd9d4 [weak]
   Release file created at: Tue, 08 Dec 2020 23:52:58 +0000
E: Some index files failed to download. They have been ignored, or old ones used instead.
Error: Process completed with exit code 100.

then later:

Run nim c -r tools/ci_testresults.nim
  nim c -r tools/ci_testresults.nim
  shell: /bin/bash --noprofile --norc -e -o pipefail {0}
  env:
    NIM_TEST_PACKAGES: 2
/home/runner/work/_temp/51f1bade-6d8f-4e0f-a401-86cd1f0307c9.sh: line 1: nim: command not found
Error: Process completed with exit code 127.