[ci] Revert fixed nightly-2021-03-23, use actual nightly

[notmandatory]

Description

Due to a regression in rust nightly 2021-04-11 which broke clang-sys our CI 'Build docs' job also broke. I hard coded this and other jobs that rely on +nightly to use +nightly-2021-03-23 instead, see #326. But docs.rs always uses +nightly so when we published 0.6.0 our docs there were still broken.

This issue is fixed in +nightly-2021-04-15 so in order to keep our CI 'Build docs' job in sync with how they will build on docs.rs I've changed back to using the actual +nightly.

Notes to the reviewers

I've also requested build 0.6.0 docs be rebuilt with the new latest +nightly, see rust-lang/docs.rs#1356.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines