Fix vcpkg command paths on Windows

[timkpaine]

Fix vcpkg command generation on Windows by removing the POSIX ./ prefix and invoking vcpkg.exe for package installation. POSIX command paths remain unchanged.

This unblocks vcpkg-enabled Windows builds in python-project-templates/python-template-cpp#86.

Verified with the full 77-test suite, ruff, mdformat, and codespell.

[github-actions]

Test Results

77 tests  +2   77 ✅ +2   1m 4s ⏱️ ±0s
 1 suites ±0    0 💤 ±0 
 1 files   ±0    0 ❌ ±0 

Results for commit ba5a14a. ± Comparison against base commit 827c49a.

[github-actions]

Test Results

77 tests  +2   77 ✅ +2   1m 0s ⏱️ -4s
 1 suites ±0    0 💤 ±0 
 1 files   ±0    0 ❌ ±0 

Results for commit ba5a14a. ± Comparison against base commit 827c49a.

♻️ This comment has been updated with latest results.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.63%. Comparing base (827c49a) to head (ba5a14a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #119      +/-   ##
==========================================
+ Coverage   94.45%   94.63%   +0.17%     
==========================================
  Files          14       14              
  Lines        1173     1192      +19     
==========================================
+ Hits         1108     1128      +20     
+ Misses         65       64       -1     

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