Disable Rustup's self update by default if the CI environment variable is detected

[rami3l]

Proposed as an alternative to #3821, mitigates #3709:

Rustup could default to not auto update if the CI environment variable is present. This is simple both to implement and explain to users.

Originally posted by @ChrisDenton in #3821 (comment)

Notes

• The relevant user guide section should be adjusted accordingly.
• This change might cause Rustup's own tests to fail, so removing CI from the environment variables is probably required before actually running those tests.

[LuXts]

Claimed.

[rami3l]

@LuXts Thanks a lot :)

[rami3l]

As there has been about a month of inactivity, plus there's #3907 already, I think it's time to unassign you. Sorry @LuXts!