Bug 1651641 - `servo/components/style/build.rs` reads `PYTHON3` configuration value in addition to environment variable r=glandium

The previous logic would check the value of the `PYTHON3` environment variable, which is not set by the Firefox build system, so this build script tends to just use the `python3` at the tip of the `PATH`, regardless of which value you've configured for it. Instead, it should prefer to read from the `OBJDIR` if one is present.

Differential Revision: https://phabricator.services.mozilla.com/D82981

config/makefiles/rust.mk

@@ -168,6 +168,7 @@ export PKG_CONFIG
 export PKG_CONFIG_ALLOW_CROSS=1
 export RUST_BACKTRACE=full
 export MOZ_TOPOBJDIR=$(topobjdir)
+export PYTHON3
 
 # Set COREAUDIO_SDK_PATH for third_party/rust/coreaudio-sys/build.rs
 ifeq ($(OS_ARCH), Darwin)