respect build requirement env vars when using native build in meson cross compilation

[planetmarshall]

Changelog: (Bugfix): Respect environment vars set in build requirements when writing the native meson configuration file in a cross compilation context. Environment vars set in a build profile still have precedence, preserving the existing behaviour.
Docs: Omit

Fixes #19214

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[planetmarshall]

Test failure in CI is due to pkg-config install failure from previous step. See #19216