Revert to install opam from debian

This is the version deployed in production (I think?) and the current (outdated) installed version in the docker image is making swh-loader-core tests fail.

Indeed:

tony@kessel:~ $ date; lsb_release -c; apt-cache show opam | grep Version
Mon Mar 10 11:54:20 AM CET 2025
No LSB modules are available.
Codename:       bookworm
Version: 2.1.2-1
swh@swh-toolbox-65cd7d565c-c8hdj:~$ opam --version
2.1.2

approved this merge request

Jenkins job jenkins-tools/swh-jenkins-dockerfiles #912 failed in 39 min.
See Console Output, Blue Ocean and Coverage Report for more details.

@jenkins retry build

Jenkins job jenkins-tools/swh-jenkins-dockerfiles #914 failed in 1 min 1 sec.
See Console Output, Blue Ocean and Coverage Report for more details.

Jenkins job jenkins-tools/swh-jenkins-dockerfiles #915 succeeded in 4 min 27 sec.
See Console Output, Blue Ocean and Coverage Report for more details.

merged