CI: Run on main and on request in addition to pull requests #188

Job	Run time
build-examples (examples/rust-hello-world, native)	2m 59s
build-tests (native)	2m 53s
build-examples (examples/rust-hello-world, sltb001a)	3m 26s
build-tests (sltb001a)	2m 58s
build-examples (examples/rust-hello-world, samr21-xpro)	3m 14s
build-tests (samr21-xpro)	2m 57s
build-examples (examples/rust-hello-world, stk3700)	3m 29s
build-tests (stk3700)	2m 58s
build-examples (examples/rust-gcoap, native)	3m 11s
build-examples (examples/rust-gcoap, sltb001a)	3m 51s
build-examples (examples/rust-gcoap, samr21-xpro)	3m 34s
build-examples (examples/rust-gcoap, stk3700)	3m 49s
build-examples (tests/rust_minimal, native)	2m 59s
build-examples (tests/rust_minimal, sltb001a)	3m 26s
build-examples (tests/rust_minimal, samr21-xpro)	3m 17s
build-examples (tests/rust_minimal, stk3700)	3m 28s
	52m 29s

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