Simplify Windows build by not setting PKG_CONFIG_PATH

[ferjm]

• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes fix #23177

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This change is 

[highfive]

Heads up! This PR modifies the following files:

• @wafflespeanut: python/servo/command_base.py, python/servo/build_commands.py

[jdm]

Looks like you made some changes after running test-tidy:

�[94m./python/servo/command_base.py�[0m:�[93m241�[0m: �[91mE302 expected 2 blank lines, found 1�[0m

�[94m./python/servo/command_base.py�[0m:�[93m259�[0m: �[91mE302 expected 2 blank lines, found 1�[0m

�[94m./python/servo/command_base.py�[0m:�[93m588�[0m: �[91mE711 comparison to None should be 'if cond is None:'�[0m

[jdm]

r? @Manishearth

[Manishearth]

@bors-servo r+

[bors-servo]

📌 Commit 34ff759 has been approved by Manishearth

[jdm]

@bors-servo r-
See previous comment.

[Manishearth]

r=me with tidy :)

[ferjm]

@bors-servo r=Manishearth

[bors-servo]

📌 Commit 7d16746 has been approved by Manishearth

[bors-servo]

⌛ Testing commit 7d16746 with merge 2e0d1ad...

[bors-servo]

☀️ Test successful - linux-rel-css, linux-rel-wpt, status-taskcluster
Approved by: Manishearth
Pushing 2e0d1ad to master...