Pin the rustc version used to compile collector

[Kobzol]

Just to avoid potential surprises with the collector being recompiled with a different (unstable) rustc every day.

[Mark-Simulacrum]

Seems ok, though I wonder if we should be trying to wire this up to dependabot or something....

[Kobzol]

If we bump MSRV in the repo, but forget to change it in the collect.sh script, we will find about that soon enough, I suppose 😆 But usually I bump it by grepping the old version, so we should see all its occurrences.