GHA: deprioritize Azure Ubuntu mirror

Due to year-long unreliability.

The default Ubuntu mirror works as fast as the Azure one when it's
working at its normal speed. And has HTTPS.

Also:
- replac the retry hack that turn out to not solve the problem.
- add timeouts to each download step to catch slowness early.

Follow-up to a5838847c4 #21181
Follow-up to 5172ba5475 #21107

Closes #21414

.github/workflows/http3-linux.yml

@@ -206,13 +206,12 @@ jobs:
 
       - name: 'install build prereqs'
         if: ${{ steps.settings.outputs.needs-build == 'true' }}
+        timeout-minutes: 2
         run: |
           sudo find /etc/apt/sources.list.d -type f -not -name 'ubuntu.sources' -delete -print
+          sudo sed -i 's/priority:1/priority:9/' /etc/apt/apt-mirrors.txt; cat /etc/apt/apt-mirrors.txt
           sudo apt-get -o Dpkg::Use-Pty=0 update
-          printf "#!/bin/sh
-            while [ \$? = 0 ]; do for i in 1 2 3; do timeout 60 \"\$@\" && break 2; echo \"Error: slow server, retry \$i\"; sleep 1
-            dpkg --configure -a; done; false; done" > "$HOME"/my-apt; chmod +x "$HOME"/my-apt
-          sudo "$HOME"/my-apt apt-get -o Dpkg::Use-Pty=0 install \
+          sudo apt-get -o Dpkg::Use-Pty=0 install \
             libtool autoconf automake pkgconf \
             libbrotli-dev libzstd-dev zlib1g-dev \
             libev-dev \
@@ -551,6 +550,7 @@ jobs:
 
     steps:
       - name: 'install prereqs'
+        timeout-minutes: 2
         env:
           INSTALL_PACKAGES: >-
             ${{ !contains(matrix.build.install_steps, 'skipall') && !contains(matrix.build.install_steps, 'skiprun') && 'stunnel4 ' || '' }}
@@ -559,11 +559,9 @@ jobs:
 
         run: |
           sudo find /etc/apt/sources.list.d -type f -not -name 'ubuntu.sources' -delete -print
+          sudo sed -i 's/priority:1/priority:9/' /etc/apt/apt-mirrors.txt; cat /etc/apt/apt-mirrors.txt
           sudo apt-get -o Dpkg::Use-Pty=0 update
-          printf "#!/bin/sh
-            while [ \$? = 0 ]; do for i in 1 2 3; do timeout 45 \"\$@\" && break 2; echo \"Error: slow server, retry \$i\"; sleep 1
-            dpkg --configure -a; done; false; done" > "$HOME"/my-apt; chmod +x "$HOME"/my-apt
-          sudo "$HOME"/my-apt apt-get -o Dpkg::Use-Pty=0 install \
+          sudo apt-get -o Dpkg::Use-Pty=0 install \
             libtool autoconf automake pkgconf \
             libpsl-dev libbrotli-dev libzstd-dev zlib1g-dev libidn2-0-dev libldap-dev libuv1-dev valgrind \
             ${INSTALL_PACKAGES} \