ci: install GTK/WebKit dev packages before setup-soldr

[zackees]

Closes #262

soldr-cook (the prebuild-deps step of cache-preset: foundation) builds
glib-sys / gobject-sys, whose build scripts resolve GTK/WebKit through
pkg-config. The apt install step was running after
zackees/setup-soldr@v0.9.62, so cooking failed with exit 101 and every run
then compiled those deps cold.

Move the Install system dependencies step ahead of setup-soldr in all six
Linux-cooking workflows: _unit-test.yml, _integration-test.yml,
_lint.yml, _terminal-test.yml, _build.yml, linux-x86-dwarf-smoke.yml.
The step body and its if: runner.os == 'Linux' guard are unchanged — this is
a pure move plus a 3-line comment explaining the ordering constraint.

In _build.yml the step is placed after Configure Rust target so
rust-toolchain.ci.toml still exists when setup-soldr reads it.

The remaining Linux-hosted setup-soldr jobs (macos-arm-live-test.yml
cross-build, macos-x64-guest-webkit-probe.yml) cross-compile to Apple targets
with a different pinned action and no foundation preset, so they never build
Linux glib-sys/gobject-sys and are left alone.

act evidence

act workflow_dispatch -W .github/workflows/linux-x86-unit-test.yml --pull=false -P ubuntu-24.04=catthehacker/ubuntu:act-24.04, run twice against the same act cache.

Run 1 (cold cache) — Job succeeded, exit 0:

⭐ Run Main Install system dependencies (Linux)
  ✅  Success - Main Install system dependencies (Linux) [1m23.397s]
⭐ Run Main zackees/setup-soldr@v0.9.62
  ✅  Success - Main zackees/setup-soldr@v0.9.62 [2m32.004s]
  ✅  Success - Main Unit Tests [1m46.976s]

01:24   Compiling glib-sys v0.18.1
01:24   Compiling gobject-sys v0.18.0
02:31 soldr cook: deps built; recipe was ephemeral
02:31 cook: completed in 102.6s
cook-cache-base       MISS      (no match)

Run 2 (warm cache) — Job succeeded, exit 0:

  ✅  Success - Main zackees/setup-soldr@v0.9.62 [21.607s]
  ✅  Success - Main Unit Tests [33.889s]

00:13 cook-cache-base: restored matched=... exact=true archive=756993772B
00:21 cook: completed in 0.4s
cook-cache-base       HIT       ...  721.9MB  9310 files

setup-soldr drops from 2m32s to 21.6s and soldr cook from 102.6s to 0.4s, i.e.
the cook now succeeds and its cache is actually reused instead of being rebuilt
cold every run.

Also verified: exactly one libwebkit2gtk-4.1-dev occurrence per file with the
apt line before the setup-soldr line in all six workflows, and every
.github/workflows/*.yml still parses as YAML.

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