CI (Buildkite, code coverage): Fail the code coverage job if the coverage is suspiciously low

[DilumAluthge]

Normally, we always pass the code coverage job.

However, if the coverage is suspiciously low (e.g. less than 50%), then this suggests that something went wrong earlier in the coverage job. Therefore, in those cases, we should fail the build to indicate that something has gone wrong somewhere in the code coverage pipeline.

[DilumAluthge]

Follow the build here: https://buildkite.com/julialang/julia-master-scheduled/builds/234

[codecov]

Codecov Report

Merging #42213 (92a3a07) into master (1709f79) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #42213      +/-   ##
==========================================
+ Coverage   89.26%   89.28%   +0.02%     
==========================================
  Files         351      351              
  Lines       78670    78664       -6     
==========================================
+ Hits        70222    70239      +17     
+ Misses       8448     8425      -23     

Impacted Files	Coverage Δ
base/secretbuffer.jl	97.82% <0.00%> (-2.18%)	⬇️
base/binaryplatforms.jl	81.76% <0.00%> (-1.52%)	⬇️
stdlib/LibGit2/src/gitcredential.jl	96.50% <0.00%> (-1.40%)	⬇️
stdlib/LibGit2/src/merge.jl	87.09% <0.00%> (-1.08%)	⬇️
base/path.jl	68.29% <0.00%> (-0.98%)	⬇️
base/filesystem.jl	90.35% <0.00%> (-0.88%)	⬇️
base/bitset.jl	93.19% <0.00%> (-0.86%)	⬇️
stdlib/LibGit2/src/LibGit2.jl	82.94% <0.00%> (-0.67%)	⬇️
base/file.jl	82.10% <0.00%> (-0.57%)	⬇️
stdlib/Random/src/generation.jl	92.88% <0.00%> (-0.45%)	⬇️
... and 23 more

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[DilumAluthge]

The coverage build (https://buildkite.com/julialang/julia-master-scheduled/builds/234) passed.