Merge bitcoin/bitcoin#27340: ci: Use Cirrus CI dockerfile env

fa4a46de0b ci: Bump nowallet_libbitcoinkernel task to ubuntu:focal (MarcoFalke)
fabc7d90a9 ci: Use credits in more tasks (MarcoFalke)
facae3b149 ci: Use Cirrus CI dockerfile env (MarcoFalke)

Pull request description:

  Currently the CI env has many intermittent issues:

  * The Ubuntu package servers are frequently down
  * Occasionally other stuff is down, such as dnf, pip, or the android sdk
  * Installing packages is slower than downloading them, at least on Cirrus, which has a fast download speed

  Fix all issues by using the Cirrus CI dockerfile env.

ACKs for top commit:
  josibake:
    code review ACK https://github.com/bitcoin/bitcoin/pull/27340/commits/fa4a46de0b3c1a5895e95dba7e95278932fbfc2c

Tree-SHA512: fea5663f7b6dc1c4ea9f87188026ec542b9269bac8ee3398cd58d4df6c86a0af9d275f1876e03f92fb1f6166ec49b817d9e588e6fe1ed54b77592502c2eccd9d

ci/test/00_setup_env_native_nowallet_libbitcoinkernel.sh

@@ -7,10 +7,9 @@
 export LC_ALL=C.UTF-8
 
 export CONTAINER_NAME=ci_native_nowallet_libbitcoinkernel
-export CI_IMAGE_NAME_TAG=debian:buster
-# Use minimum supported python3.7 and clang-8, see doc/dependencies.md
-export PACKAGES="-t buster-backports python3-zmq clang-8 llvm-8 libc++abi-8-dev libc++-8-dev"
-export APPEND_APT_SOURCES_LIST="deb http://deb.debian.org/debian buster-backports main"
+export CI_IMAGE_NAME_TAG=ubuntu:focal
+# Use minimum supported python3.7 (or python3.8, as best-effort) and clang-8, see doc/dependencies.md
+export PACKAGES="python3-zmq clang-8 llvm-8 libc++abi-8-dev libc++-8-dev"
 export DEP_OPTS="NO_WALLET=1 CC=clang-8 CXX='clang++-8 -stdlib=libc++'"
 export GOAL="install"
 export NO_WERROR=1