Fix compatibility with master cluster mode

[lkubb]

What does this PR do?

Fix compatibility with master cluster mode

What issues does this PR fix or reference?

Fixes: #99

Previous Behavior

Crash because the pki_dir does not contain the minion's key, it's in cluster_pki_dir.

New Behavior

Works.

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

Commits signed with GPG?

Yes

[codecov]

Codecov Report

Attention: Patch coverage is 90.52632% with 9 lines in your changes missing coverage. Please review.

Project coverage is 90.83%. Comparing base (0e4d84e) to head (9a44551).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...s/integration/runners/test_vault_master_cluster.py	87.30%	8 Missing ⚠️
src/saltext/vault/runners/vault.py	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #100      +/-   ##
==========================================
+ Coverage   90.78%   90.83%   +0.05%     
==========================================
  Files          77       79       +2     
  Lines       13147    13231      +84     
  Branches     1219     1224       +5     
==========================================
+ Hits        11935    12018      +83     
  Misses        953      953              
- Partials      259      260       +1     

Flag	Coverage Δ
Linux	90.75% <90.52%> (+0.05%)	⬆️
macOS	66.40% <56.84%> (-0.01%)	⬇️
project	80.49% <85.71%> (+0.17%)	⬆️
py310	90.78% <90.52%> (+0.05%)	⬆️
py39	85.47% <61.05%> (-0.13%)	⬇️
salt_3006_9	85.54% <61.05%> (-0.15%)	⬇️
salt_3007_1	90.74% <90.52%> (+0.05%)	⬆️
tests	95.99% <90.90%> (-0.06%)	⬇️

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