Pin the toolchain version in the tree

[iderex]

A toolchain that is whatever the runner image happens to ship is a build that
changes without a commit. It also makes a contributor's local failure impossible
to compare with a gate failure, because the two ran different compilers.

Done when

A version file in the tree names the exact toolchain version, the gate reads that
file rather than carrying a duplicate version string in workflow YAML, and a
contributor with the wrong version installed is told so by name and number rather
than by a compile error. Blocked on #11.

[iderex]

The record this issue waits on is on the default branch, and it deliberately pins
no version, so what is left here is exactly the pin.

git ls-tree --name-only origin/main docs/decisions/ | grep 0011
docs/decisions/0011-the-language-the-toolchain-and-the-binding-layer.md

The record's own sentence about it is that a version written into a decision
record would be a second declaration of the same fact, and the two would disagree
at the first upgrade. So the digit belongs here and in one file in the tree.

What the answer fixes for this issue: the thing pinned is a Rust toolchain, and
the components the gate needs are components of it rather than separate installs,
which is why one pin covers the analyser in #17 and the formatter in #18 as well
as the compiler:

rustup component list --installed
cargo-x86_64-pc-windows-msvc
clippy-x86_64-pc-windows-msvc
rust-docs-x86_64-pc-windows-msvc
rust-std-x86_64-pc-windows-msvc
rustc-x86_64-pc-windows-msvc
rustfmt-x86_64-pc-windows-msvc

One thing worth having here before the pin is chosen. The detector in #117 is a
nightly compiler flag, so a single pinned stable version does not cover that leg,
and whether the second toolchain is pinned in the same file or named where the leg
runs is a question this issue is the right place for. The record carries the
readings behind that.

This comment carries the answer and nothing else. #11's done-condition is that
every M2 issue naming it as a blocker has been updated with what it was waiting
for, and this is that update; the pin itself and this issue's labels are left as
they are.