CI fixes

[ellie-idb]

Part of #16 (with parts of #12) mixed in.

[codecov-commenter]

Codecov Report

Merging #20 (c55546c) into master (ec28d7e) will increase coverage by 0.29%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
+ Coverage   86.28%   86.57%   +0.29%     
==========================================
  Files          32       33       +1     
  Lines        7202     7479     +277     
==========================================
+ Hits         6214     6475     +261     
- Misses        988     1004      +16     

Impacted Files	Coverage Δ
source/mir/ion/tape.d	94.14% <0.00%> (-3.63%)	⬇️
source/mir/ion/value.d	71.16% <0.00%> (-0.59%)	⬇️
source/mir/ion/internal/stage3.d	100.00% <0.00%> (ø)
source/mir/ser/json.d	95.59% <0.00%> (+0.02%)	⬆️
source/mir/ion/examples.d	94.05% <0.00%> (+0.06%)	⬆️
source/mir/deser/package.d	73.13% <0.00%> (+0.15%)	⬆️
source/mir/ion/internal/stage1.d	95.94% <0.00%> (+0.42%)	⬆️
source/mir/ion/internal/stage2.d	73.25% <0.00%> (+27.37%)	⬆️

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[ellie-idb]

Investigating how to best do ARM runners (I generally want to avoid using CircleCI). TBD.