Enable CRB repo on alma9, PowerTools on alma8

[h-vetinari]

With the switch to alma8 & alma9, some of our CDTs are now split across separate repos. In particular, we need PowerTools on alma8, resp. CRB on alma9 (which is essentially the same thing but got renamed). See for example conda-forge/cdt-builds@e118347

As an example, the following failed:

+ /usr/bin/sudo -n yum install -y wget m4 help2man patch gcc-gfortran gcc-c++ rsync sed findutils make file strace
AlmaLinux 9 - AppStream                          30 MB/s | 9.2 MB     00:00    
AlmaLinux 9 - BaseOS                            3.2 MB/s | 4.5 MB     00:01    
AlmaLinux 9 - Extras                             86 kB/s |  13 kB     00:00    
No match for argument: help2man

That package is available under CRB on alma9, so enable this repo.

[h-vetinari]

I'm fine for this PR to get squashed, if someone prefers that. I didn't squash the PR myself to show why I arrived at the solution here. Note that the following sed-command

docker-images/scripts/fix_rpm

Line 7 in 7023391

sed -i '/^enabled=/d; /^\[/a\enabled=0' /etc/yum.repos.d/CentOS-Base.repo

is pretty gnarly to understand, and in particular, I couldn't find a way to chain another append-command afterwards.

I added some comments to explain a bit what's going on. Whereas the regex for cos7 unconditionally disables something (so adding enable=0 everywhere is fine), here we only want to activate CRB resp. PowerTools, but without their sources or debuginfo flavours; hence we need two append commands.

[jaimergp]

This LGTM but I'd prefer if someone else can also take a look before merging because I'm not that in touch with this part of the stack.

[h-vetinari]

Ping @conda-forge/core, would be nice if someone could review. :)