GHA/linux-old: add support for using custom CMake versions

Install CMake from the Kitware GitHub release archive. To allow choosing
its version independently from the OS.

Switch to 3.7.0 (from 3.7.2) to test the earliest supported version.
Also tested OK with 3.18.4 and 3.7.2.

The download and install step takes 1-2 seconds.

Follow-up to c9e50e9e39 #19737

Closes #19738

.github/workflows/linux-old.yml

@@ -60,6 +60,8 @@ jobs:
     runs-on: 'ubuntu-latest'
     container: 'debian:stretch'
 
+    env:
+      CMAKE_VERSION: '3.7.0'  # Earliest version supported by curl
     steps:
       - name: 'install prereqs'
         # Remember, this shell is dash, not bash
@@ -72,7 +74,7 @@ jobs:
           dpkg -i freexian-archive-keyring_2022.06.08_all.deb
           echo 'deb http://deb.freexian.com/extended-lts stretch-lts main contrib non-free' | tee /etc/apt/sources.list.d/extended-lts.list
           apt-get -o Dpkg::Use-Pty=0 update
-          apt-get -o Dpkg::Use-Pty=0 install -y --no-install-suggests --no-install-recommends cmake make automake autoconf libtool gcc pkg-config libpsl-dev libzstd-dev zlib1g-dev libc-ares-dev libkrb5-dev libldap2-dev librtmp-dev stunnel4
+          apt-get -o Dpkg::Use-Pty=0 install -y --no-install-suggests --no-install-recommends make automake autoconf libtool gcc pkg-config libpsl-dev libzstd-dev zlib1g-dev libc-ares-dev libkrb5-dev libldap2-dev librtmp-dev stunnel4
           # GitHub's actions/checkout needs newer glibc and libstdc++. The latter also depends on
           # gcc-8-base, but it does not actually seem used in our situation and is not available in
           # the main repo, so force the install.
@@ -80,6 +82,15 @@ jobs:
           httrack --get https://deb.freexian.com/extended-lts/pool/main/g/gcc-8/libstdc++6_8.3.0-6_amd64.deb
           dpkg -i --force-depends libc6_*_amd64.deb libstdc++6_*_amd64.deb
 
+      - name: 'install prereqs (cmake)'
+        run: |
+          cd ~
+          fn="cmake-${CMAKE_VERSION}-linux-x86_64"
+          httrack --get "https://github.com/Kitware/CMake/releases/download/v${CMAKE_VERSION}/${fn}.tar.gz"
+          tar -xf "${fn}".tar*.gz
+          rm -f "${fn}".tar*.gz
+          mv "cmake-${CMAKE_VERSION}-Linux-x86_64" cmake
+
       - uses: actions/checkout@1af3b93b6815bc44a9784bd300feb67ff0d1eeb3 # v6.0.0
         with:
           persist-credentials: false
@@ -88,7 +99,7 @@ jobs:
         run: |
           mkdir bld-1
           cd bld-1
-          cmake .. -DCMAKE_UNITY_BUILD=ON -DCURL_WERROR=ON -DBUILD_SHARED_LIBS=ON \
+          "$HOME"/cmake/bin/cmake .. -DCMAKE_UNITY_BUILD=ON -DCURL_WERROR=ON -DBUILD_SHARED_LIBS=ON \
             -DCURL_ENABLE_SSL=OFF -DENABLE_ARES=OFF -DCURL_ZSTD=OFF -DCURL_USE_GSSAPI=OFF -DCURL_USE_LIBSSH2=OFF -DCURL_USE_LIBSSH=OFF -DUSE_LIBRTMP=ON
 
       - name: 'CM build-only build'
@@ -112,7 +123,7 @@ jobs:
         run: |
           mkdir bld-cares
           cd bld-cares
-          cmake .. -DCMAKE_UNITY_BUILD=ON -DCURL_WERROR=ON -DBUILD_SHARED_LIBS=ON \
+          "$HOME"/cmake/bin/cmake .. -DCMAKE_UNITY_BUILD=ON -DCURL_WERROR=ON -DBUILD_SHARED_LIBS=ON \
             -DCURL_ENABLE_SSL=OFF -DENABLE_ARES=ON -DCURL_USE_GSSAPI=ON -DCURL_USE_LIBSSH2=OFF -DCURL_USE_LIBSSH=OFF -DUSE_LIBRTMP=ON \
             -DCURL_LIBCURL_VERSIONED_SYMBOLS=ON