Skip installing pip from weak deps

[am11]

Fedora 34 build started to fail recently https://dev.azure.com/dnceng/public/_build/results?buildId=1356807&view=logs&j=713d3e5f-f6a4-56d6-79f4-bba7fa11838c&t=8bb88579-afd7-5860-9e3c-89c6414081a4&l=1927:

Installing collected packages: wheel, pip
  Attempting uninstall: pip
    Found existing installation: pip 21.0.1
ERROR: Cannot uninstall pip 21.0.1, RECORD file not found. Hint: The package was installed by rpm.

apparently until yesterday it was fine .. https://dev.azure.com/dnceng/public/_build/results?buildId=1350156&view=logs&j=713d3e5f-f6a4-56d6-79f4-bba7fa11838c

Seems like maintainers have recently marked python-pip as an optional/weak dependency:

2021-09-11T07:14:22.4929004Z Installing weak dependencies:
2021-09-11T07:14:22.4929613Z  compiler-rt                 x86_64  12.0.1-1.fc34               updates  2.3 M
2021-09-11T07:14:22.4930291Z  gcc-gdb-plugin              x86_64  11.2.1-1.fc34               updates  143 k
2021-09-11T07:14:22.4930942Z  libomp                      x86_64  12.0.1-1.fc34               updates  353 k
2021-09-11T07:14:22.4931617Z  libomp-devel                x86_64  12.0.1-1.fc34               updates   24 k
2021-09-11T07:14:22.4932285Z  libxcrypt-compat            x86_64  4.4.25-1.fc34               updates   89 k
2021-09-11T07:14:22.4932941Z  mkpasswd                    x86_64  5.5.10-1.fc34               updates   41 k
2021-09-11T07:14:22.4933613Z  python3-pip                 noarch  21.0.1-3.fc34               updates  1.8 M
2021-09-11T07:14:22.4933892Z 
2021-09-11T07:14:22.4934559Z Transaction Summary
2021-09-11T07:14:22.4934952Z ================================================================================

(python3-pip is new between yesterday's logs and today's logs)

Fix is to skip weak deps, and explicitly specify stuff we need. Just to be on the safe side, I have kept libomp-dev as it's used by ML.NET, however we only explicitly require it on Ubuntu (see, e.g. rootfs setup https://github.com/dotnet/arcade/blob/054309cb477a969df1197b9286a625827217af5f/eng/common/cross/build-rootfs.sh#L78-L80).