manual: bump mdbook to 0.5.x and replace mdbook-environment env-var interpolation

[schell]

Problem

manual/book.toml declares the [preprocessor.environment] preprocessor (the
mdbook-environment crate) to
interpolate {{DOCS_URL}} (and other env vars) into the manual's markdown.
mdbook-environment 0.0.4 (latest, built against mdbook ^0.4) cannot parse
mdbook 0.5's preprocessor input, so with mdbook 0.5.x the manual build fails:

INFO Book building has started
Unable to parse the input
ERROR The "environment" preprocessor exited unsuccessfully with exit status: 1

Because xtask's Manual::install_deps installs mdbook unpinned
(cargo install mdbook), a fresh machine now gets mdbook 0.5.x (latest 0.5.4)
and the manual cannot build. Downstream, the renderling.xyz site pipeline then
fails with a confusing manual/book canonicalize panic.

Workaround currently in place: the deploy machine is pinned to
mdbook 0.4.52 (cargo install mdbook --version 0.4.52 --force). The wgsl-rs
book (which uses no external preprocessors) builds fine on both 0.4 and 0.5.

Ask

1. Bump to mdbook 0.5.x and replace the env-var interpolation with something
   that works there. Ideas: vendor/patch mdbook-environment against 0.5 (or
   upstream a fix), generate the docs URL into an {{#include}}d file at
   build time from xtask (which already receives --docs-url), or
   preprocess the sources before invoking mdbook.
2. Pin the mdbook version in Manual::install_deps either way, so a fresh
   machine cannot silently drift onto an incompatible version.

Related

• deps.rs has_binary probes for binaries by shelling out to hash, a shell
  builtin — a fragile check.
• When the manual build fails, xtask logs could not build the manual but
  exits 0, so callers believe the build succeeded and fail later with an
  unrelated-looking panic. The build failure should exit non-zero.

Found while deploying the wgsl-rs operator's manual through the renderling.xyz
pipeline (wgsl-rs#29).