Snowflake bridge parameters are too long (535 bytes) in 11.5.5

The bridge line for snowflake in Tor Browser 11.5.5 is too long to fit in the pluggable transports SOCKS username:password encoding. Attempting to use the default snowflake configuration fails with the following warnings. tor proceeds to act as if no bridges were configured, starts connecting to ordinary relays, and chooses one as a guard.

[WARN] SOCKS arguments can't be more than 510 bytes (535).
[WARN] Controller gave us config lines that didn't validate: Bridge line did not parse. See logs for details.

!540 (closed) is the merge request that made the line too long. To be more precise, the bridge line proposed in tpo/anti-censorship/team#101 (closed), on its own, would have been okay (502 bytes); it was that plus the addition of utls-imitate=... that made it altogether too long.