Prevent failing to restart setup-toolchain
Prevent failing to restart setup-toolchain

If `rustup-toolchain-install-master` fails to install master
toolchain (in case `rustup update` executes at the same remove the
tmp directory), `cargo search` (which use master toolchain after
`rustup override` command) will fail. This leads to setup-toolchain
fails too.

graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust