Drop support for DEPUTY_IPC_DIR env var

[omus]

Uses Mocking.jl to override the IPC directory for testing. Follow up to #13

[omus]

I'll need to look into the Julia 1.6 failure to proceed here

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.31%. Comparing base (b62e185) to head (699b505).

Files	Patch %	Lines
src/graceful_termination.jl	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #14      +/-   ##
==========================================
- Coverage   89.61%   88.31%   -1.30%     
==========================================
  Files           4        4              
  Lines          77       77              
==========================================
- Hits           69       68       -1     
- Misses          8        9       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.