Use safe march flags for AUTOBAHN

Setting `AUTOBAHN_ARCH_TARGET=safe` will determine the autobahn nvx sources
to be compiled with save march flags as per:
https://github.com/crossbario/autobahn-python/blob/1c7d45cd2129b1fa6daacb0ed7051448fed3ec0d/src/autobahn/nvx/_compile_args.py#L215

In our scenario the `RH10 ARM` build compiled
`_utf8validator` with `PAC` support, being tricked by the QEMU emulation under buildx.
`PAC` was introduced on `ARMv8.3-A` while our arm hosts are `ARMv8.2-A`

The CI built image passes the GH Actions tests under emulation, while
on our ARM hosts we can see that it crashes when trying to push the `RETAA` instruction,
during a `buildbot-worker` startup.

```
[buildbot@6d98845070da ~]$ twistd --pidfile= -ny /home/buildbot/buildbot.tac
Illegal instruction (core dumped)

=> 0xfffedeb72120 <PyInit__nvx_utf8validator+184>:  retaa
```

Author: RazvanLiviuVarzaru

ci_build_images/pip.Dockerfile

@@ -6,6 +6,7 @@
 # Install a recent rust toolchain needed for some arch
 # see: https://cryptography.io/en/latest/installation/
 # then upgrade pip and install BB worker requirements
+ENV AUTOBAHN_ARCH_TARGET=safe
 RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs >/tmp/rustup-init.sh \
     # rust installer does not detect i386 arch \
     && case $(getconf LONG_BIT) in \