Set warn_non_default_branch = true in triagebot.toml #2558

Job	Run time
cargo +1.56 build	1m 36s
cargo bench	45s
cargo +1.36 build (futures-{core, io, sink})	1m 7s
cargo hack check --feature-powerset	2m 20s
cargo +stable build	47s
cargo +beta build	40s
cargo minimal-versions build	27s
cargo build --target thumbv7m-none-eabi	28s
cargo +nightly build	40s
cargo test (ubuntu-latest)	3m 26s
cargo build --target thumbv6m-none-eabi	28s
cargo test (macos-latest)	5m 32s
cargo test (windows-latest)	4m 46s
cargo clippy	30s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 26s
cargo doc	17s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 3s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 11s
cargo fmt	7s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 26s
cargo miri test	9m 34s
cargo test -Z sanitizer=address	3m 34s
cargo test -Z sanitizer=memory	3m 56s
cargo test -Z sanitizer=thread	2m 33s
	1h 2m 39s

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