attempt switch from nyc to c8

[cspotcode]

I've done a bunch of nyc support hacks in our tests, and yet there are still weird issues.

I'm hopeful that switching to c8 avoids all that.

[codecov]

Codecov Report

Merging #1816 (c090710) into main (97f9afd) will increase coverage by 5.40%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
src/bin-cwd.ts	100.00% <0.00%> (ø)
src/bin-esm.ts	100.00% <0.00%> (ø)
src/bin-script.ts	100.00% <0.00%> (ø)
dist-raw/runmain-hack.js	100.00% <0.00%> (ø)
src/child/argv-payload.ts	100.00% <0.00%> (ø)
dist-raw/node-nativemodule.js	100.00% <0.00%> (ø)
dist-raw/node-internal-constants.js	100.00% <0.00%> (ø)
dist-raw/node-internal-modules-cjs-helpers.js	100.00% <0.00%> (ø)
register/transpile-only.js	0.00% <0.00%> (ø)
register/files.js	0.00% <0.00%> (ø)
... and 36 more