fix(ci): Install rustup when creating docker environment (#7871)

* fix(ci): Install rustup when creating docker environment

This was removed in e147113ba as our CI runners already have this
installed, but the bootstrap script is also shared with the `Dockerfile`
used to create the image used by `make environment`.

This is related to https://github.com/timberio/vector/issues/7862 which
should hopefully improve the bootstrapping situation.

Failure: https://github.com/timberio/vector/runs/2833564450#step:6:8338

Signed-off-by: Jesse Szwedko <jesse@szwedko.me>

* Add missing release-flags script

Signed-off-by: Jesse Szwedko <jesse@szwedko.me>

* Run environment workflow on PRs too

But only push on master still

Signed-off-by: Jesse Szwedko <jesse@szwedko.me>