fix: use gnu sed in shell tests for cross-platform compatibility

[montchr]

Darwin's use of BSD sed breaks here because BSD sed has a different
argument signature.

Spun off from #438 as suggested in #438 (comment)

[Pacman99]

LGTM
you should be able to bors r+

[montchr]

@Pacman99 I opted for a PR mainly to go through the exercise of learning about the CI/build process for PRs. What steps do I take to run a CI build for this branch? You got there first! I haven't had enough coffee yet 🙃

[montchr]

bors r+

[montchr]

@Pacman99 anything in addition to the details in the bors getting started guide i should know about the process specific to divnix and digga?

[bors]

Build failed:

• check

[Pacman99]

Not that I can think of, i think you know everything about where checks are located. There is also bors delegate+ for giving PR authors ability to merge their own PRs once they get an up to date approval.

bors try

[montchr]

Strange that builds are hanging on this one… I cancelled the first run once it had been running for an hour. I stepped away and came back to find this build has been running for two hours…

[Pacman99]

Yeah I'm not sure whats up with that 😕
This one only took 8 min: https://github.com/divnix/digga/runs/5413509528?check_suite_focus=true

And this PR doesn't change anything significant to change build times by that much

[bors]

try

Build failed:

• check

[montchr]

bors try

[bors]

try

Build succeeded:

• check

[montchr]

bors r+

[bors]

Build succeeded:

• check