refactor: Use compilation target instead of build machine target for MSRV checks

[foresterre]

Instead of installing target toolchain directly, add the target toolchain as a target to the build machine toolchain.

The first commit does not attempt to address the same issue for the check, it only refactors the way toolchains and targets are installed. Next up will be ensuring that the target is included (at least by default) with cargo check --target <target>

closes #824
closes #587

[codecov]

Codecov Report

Attention: 156 lines in your changes are missing coverage. Please review.

Comparison is base (06864c7) 75.73% compared to head (d2e571f) 75.22%.
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #825      +/-   ##
==========================================
- Coverage   75.73%   75.22%   -0.51%     
==========================================
  Files          81       81              
  Lines        6057     6118      +61     
==========================================
+ Hits         4587     4602      +15     
- Misses       1470     1516      +46     

Files	Coverage Δ
src/sub_command/find.rs	87.89% <100.00%> (ø)
src/sub_command/find/tests.rs	100.00% <100.00%> (ø)
src/cli/custom_check_opts.rs	50.00% <0.00%> (+50.00%)	⬆️
src/context/find.rs	14.81% <80.00%> (+14.81%)	⬆️
src/lib.rs	0.00% <0.00%> (ø)
src/command.rs	0.00% <0.00%> (ø)
src/context.rs	17.58% <0.00%> (-1.14%)	⬇️
src/context/verify.rs	0.00% <0.00%> (ø)
src/download.rs	0.00% <0.00%> (ø)
src/check/rustup_toolchain_check.rs	8.07% <12.50%> (+8.07%)	⬆️

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