[ric-oliv]

Every Linux and macOS job that uses Setup Environment installs Mono, but nothing runs it anymore.

• ci: install Mono for Ubuntu (ARM64) #4545 added Mono on Ubuntu for one integration test, Ensure System.Reflection.Metadata is not missing. It runs the net4-console app, which on Linux and macOS goes through Mono.
• Two weeks later Add support for net10.0-rc.1 #4461 made that test Windows-only with -Skip:(!$IsWindows). In the logs of ci: measure the round 4 CI changes together (do not merge) #5788 it shows as skipped on linux-x64, linux-arm64 and macOS, and nothing else launches mono.
• The .NET Framework unit tests only target net48 on Windows. The crash tests start the crashable app through dotnet on every other target.

On Ubuntu the install downloads 53 MB from azure.archive.ubuntu.com. On October 23 that download crawled at 43 to 59 kB/s and took 15 to 21 minutes in 4 linux-x64 legs and 1 Android device test job. It made linux-x64 the slowest leg on #5791, and an Android run took 41 minutes on #5793.

This PR removes both Mono steps from Setup Environment:

• apt install mono-devel on Ubuntu, together with the chmod that only repaired the /usr/share/dotnet permissions Mono's install had changed,
• brew install mono on macOS.

In #5788's logs the Mono step took 15 to 18 seconds on macOS jobs, 30 to 55 seconds on Linux jobs, and 403 seconds on its linux-x64 leg. So this saves under a minute on a good run, and up to 20 minutes when the mirror is slow.

Measured on the first run. Every job passed, so setup-dotnet doesn't need the chmod.

	Before	This PR
Android Device Tests	median 45.8 min over the last 10 PR runs, 28.9 to 57.3	27.4
Setup Environment in the Android test jobs	~3.5, but 9 to 26 in 11 of 40 jobs, in 7 of those 10 runs	2.8 to 3.0
Setup Environment in the Android build jobs	2.6 to 3.3	2.5 to 2.6
Setup Environment on linux-x64	~3.3, but 9 to 23.5 in 7 of the previous 19 build runs	2.7
Setup Environment on macOS	2.8 to 3.8	2.5

#5795 moves the Android test jobs from the ubuntu-latest-4-cores larger runner to ubuntu-latest.

The build workflow took 49.4 minutes on this run, because win-arm64 drew a slow host: its Build step took 13 minutes, against about 8 normally. That's unrelated to this change.

#skip-changelog

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.12%. Comparing base (b41b96c) to head (532260d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5794      +/-   ##
==========================================
- Coverage   76.13%   76.12%   -0.02%     
==========================================
  Files         528      528              
  Lines       19579    19579              
  Branches     3829     3829              
==========================================
- Hits        14906    14904       -2     
- Misses       3791     3792       +1     
- Partials      882      883       +1     

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[ric-oliv]

I'm continuing this in #5807, part 1 of 6 in #5806, so the CI changes can be reviewed in a few steps instead of 13 PRs. The measurements here stay as the record.