Add ghcup step

[9999years]

This upgrades ghcup: https://www.haskell.org/ghcup/

Potential work: if stack is installed through ghcup, you're not supposed to run stack upgrade, so I could potentially patch this to ignore the stack step when ghcup is present. (And/or detect if stack is a ghcup-installed stack.)

Standards checklist:

• The PR title is descriptive.
• The code compiles (cargo build)
• The code passes rustfmt (cargo fmt)
• The code passes clippy (cargo clippy)
• The code passes tests (cargo test)
  • Yes but it ran zero tests? Also I didn't write any
• Optional: I have tested the code myself
  • I also tested that Topgrade skips the step where needed

[codecov]

Codecov Report

Merging #89 (604dd74) into master (9cd155f) will decrease coverage by 0.14%.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##           master     #89      +/-   ##
=========================================
- Coverage    3.50%   3.35%   -0.15%     
=========================================
  Files          27      27              
  Lines        3141    3280     +139     
=========================================
  Hits          110     110              
- Misses       3031    3170     +139     

Impacted Files	Coverage Δ
src/config.rs	0.00% <0.00%> (ø)
src/main.rs	0.42% <0.00%> (-0.01%)	⬇️
src/steps/containers.rs	0.00% <0.00%> (ø)
src/steps/emacs.rs	0.00% <0.00%> (ø)
src/steps/generic.rs	0.00% <0.00%> (ø)
src/steps/git.rs	0.00% <0.00%> (ø)
src/steps/kakoune.rs	0.00% <0.00%> (ø)
src/steps/node.rs	0.00% <0.00%> (ø)
src/steps/os/archlinux.rs	0.00% <0.00%> (ø)
src/steps/os/linux.rs	22.29% <0.00%> (-1.01%)	⬇️
... and 9 more

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