install-deps: pin delta build to newer GCC on Ubuntu 20.04 arm64

[Nox-MSFT]

The delta library's src/native/build.sh hard-codes triplet-prefixed compiler paths (e.g. /usr/bin/aarch64-linux-gnu-g++) and passes them to CMake via -DCMAKE_C_COMPILER / -DCMAKE_CXX_COMPILER, ignoring the CC/CXX env vars and the unprefixed update-alternatives we already set.

On Ubuntu 20.04 arm64 the default aarch64-linux-gnu-g++ is GCC 9, which lacks the C++20 header that the delta sources require, so the build fails with: fatal error: span: No such file or directory.

Fix: patch the cloned build.sh to honor CC/CXX as fallback defaults, and set CC/CXX (scoped to the child process) only when the triplet's default g++ is actually older than the gcc_ver we just installed. This leaves debian11-arm64 (g++ 10), ubuntu2204-arm64 (g++ 11) and amd64 (already aliased via alternatives) untouched.

[Nox-MSFT]

Rebased onto latest develop

Force-pushed a rebase of this branch onto current develop (was ~10 commits behind). The branch now also contains:

• ci(docker-build): retry apt steps to absorb transient mirror failures #885 ("retry apt steps to absorb transient mirror failures"), and
• the ubuntu:20.04 + arm64 CI exclusion added on develop.

About the earlier container-job (debian:11, arm64) failure

That failure was not a code/compiler problem — it was a transient apt mirror error while fetching gdb:

Err:9 ... gdb arm64 10.1-1.7  Error reading from server - read (104: Connection reset by peer)
E: Failed to fetch .../gdb_10.1-1.7_arm64.deb

debian:11 ships g++ 10, which already supports the C++20 <span> this PR is about, so debian11-arm64 is a healthy target. The job failed only because this branch predated #885's apt-retry hardening. The rebase pulls that in, so the re-triggered run should be resilient to the flaky download. (No need to drop debian:11, arm64 from the matrix.)

Note on scope — merge vs. supersede

develop's CI now excludes ubuntu:20.04 + arm64 (the exact combination this PR fixes), citing the GCC‑9/<span> issue and Ubuntu 20.04 being EOL (April 2025). As a result, after the rebase this PR has no effect on CI; it only benefits manual/local delta-handler builds on Ubuntu 20.04 arm64.

Two coherent options for maintainers:

• Merge — to keep manual Ubuntu 20.04 arm64 delta builds working, or
• Close as superseded — if Ubuntu 20.04 arm64 is no longer a supported target given the EOL exclusion.