tiup: `update --self` need to add some judgments.

[srstack]

Feature Request

Is your feature request related to a problem? Please describe:

If TiUP is not installed in ${TiUP_HOME}/bin, update --self will fail, so we need to add some judgments.

• brew install pingcap/brew/tiup

Describe the feature you'd like:

If TiUP is not installed in ${TiUP_HOME}/bin, there will be information telling users to upgrade TiUP by installing TiUP.

Why the featue is needed:

if i use brew install tiup, i use tiup update --self will fail and i don't know why.

tiup update --self
Backup of `/Users/srstack/.tiup/bin/tiup` to `/Users/srstack/.tiup/bin/tiup.tmp` failed.
Error: rename /Users/srstack/.tiup/bin/tiup /Users/srstack/.tiup/bin/tiup.tmp: no such file or directory

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy: