Set SCCACHE_IDLE_TIMEOUT=0 when starting the sccache server

[luser]

This will prevent the sccache server from shutting down due to a lack of activity which can manifest as build failures. This is effectively a workaround for this sccache issue: mozilla/sccache#204 .

---

This change is 

[staktrace]

Thanks, this is good to know, but we're also still hitting #3248 (comment) so at the moment I'd still like to leave sccache disabled until it's more reliable.

(Also note that the .taskcluster.yml file has two different jobs which use sccache so both of them would need this fix applied.)

[bors-servo]

☔ The latest upstream changes (presumably #3402) made this pull request unmergeable. Please resolve the merge conflicts.

[gw3583]

@staktrace @luser Do we want to merge this (after a rebase) or should this be closed?

[staktrace]

Unless we know how to resolve #3248 (comment) I don't think it's worth re-enabling sccache. So I'd lean towards closing this for now.

[gw3583]

OK, feel free to reopen if we want to reconsider landing this.