src: call unmask after install signal handler

[theanarkh]

Call unmask after installing signal handler to ensure the signal is caught.

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

[nodejs-github-bot]

Review requested:

• @nodejs/inspector

[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 90.05%. Comparing base (049664b) to head (a6c462c).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/inspector_agent.cc	33.33%	1 Missing and 1 partial ⚠️
src/node_watchdog.cc	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #59059      +/-   ##
==========================================
- Coverage   90.06%   90.05%   -0.01%     
==========================================
  Files         645      645              
  Lines      189130   189132       +2     
  Branches    37094    37095       +1     
==========================================
- Hits       170339   170331       -8     
+ Misses      11511    11506       -5     
- Partials     7280     7295      +15     

Files with missing lines	Coverage Δ
src/node_watchdog.cc	71.94% <50.00%> (+0.12%)	⬆️
src/inspector_agent.cc	79.94% <33.33%> (-0.11%)	⬇️

... and 25 files with indirect coverage changes

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