gn: leave PKG_CONFIG_PATH untouched when not using a sysroot
gn: leave PKG_CONFIG_PATH untouched when not using a sysroot

This makes cross-compilation possible when not using a sysroot.

Review URL: https://codereview.chromium.org/1543483002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#367906}

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