Fix a race when registering via email 3pid #2399

Job	Run time
check-lockfile	6s
lint-crlf	3s
changes	7s
lint-newsfile	33s
lint	53s
Typechecking	1m 42s
check-schema-delta	41s
check-sampleconfig	41s
lint-pydantic	45s
lint-clippy	0s
lint-clippy-nightly	0s
lint-readme	0s
lint-rustfmt	0s
linting-done	7s
calculate-test-jobs	8s
trial-olddeps	7m 44s
complement (monolith, SQLite)	12m 43s
portdb (3.8, 11)	3m 5s
complement (monolith, Postgres)	13m 33s
portdb (3.11, 15)	3m 3s
cargo-test	0s
complement (workers, Postgres)	20m 50s
trial-pypy	0s
cargo-bench	0s
sytest (focal)	8m 48s
sytest (focal, postgres)	10m 21s
trial (3.8, sqlite, all)	4m 54s
sytest (focal, multi-postgres, workers)	13m 42s
trial (3.8, postgres, 11, all)	11m 34s
sytest (focal, multi-postgres, workers, asyncio)	20m 32s
trial (3.8, sqlite)	4m 37s
export-data	2m 30s
tests-done	4s
	2h 23m 46s

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