CI: Run on main and on request in addition to pull requests #196

Job	Run time
build-tests (native)	2m 38s
build-examples (examples/rust-hello-world, native)	3m 4s
build-tests (sltb001a)	2m 40s
build-examples (examples/rust-hello-world, sltb001a)	3m 37s
build-tests (samr21-xpro)	2m 35s
build-examples (examples/rust-hello-world, samr21-xpro)	3m 13s
build-tests (stk3700)	2m 29s
build-examples (examples/rust-hello-world, stk3700)	3m 25s
build-examples (examples/rust-gcoap, native)	3m 18s
build-examples (examples/rust-gcoap, sltb001a)	3m 50s
build-examples (examples/rust-gcoap, samr21-xpro)	3m 41s
build-examples (examples/rust-gcoap, stk3700)	3m 48s
build-examples (tests/rust_minimal, native)	3m 0s
build-examples (tests/rust_minimal, sltb001a)	3m 35s
build-examples (tests/rust_minimal, samr21-xpro)	3m 19s
build-examples (tests/rust_minimal, stk3700)	3m 27s
	51m 39s

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