meta-rust does internet access in do_compile

upstream meta-rust issue:
https://github.com/meta-rust/meta-rust/issues/87

doing internet access in do_compile gives issues with corporate environment, which require http_proxy. This will also probably give build instabilities as sstate is not expecting internet access (and thus potentially changing inputs) in do_compile.

related upstream issues:
https://github.com/meta-rust/meta-rust/pull/83
https://github.com/meta-rust/meta-rust/issues/84

A solution for proxy beyond my #83 is potentially Chameleonsocks, which I couldn't test as it requires root access on the build servers. See option 2 of:
https://wiki.yoctoproject.org/wiki/Working_Behind_a_Network_Proxy