Bump the workflow image pin to the 2026.06.14 tag that bakes in
wasm-tools. And keep the wasm-tools download/caching steps for the macOS
and Windows jobs — but skip those steps on Linux. So, the Linux jobs use
the wasm-tools copy the image installs.
Follow-up to https://github.com/LadybirdBrowser/ladybird/pull/9972
The Linux CI workflows were still pinned to ladybird-ci:2026.05.4, even
though the CI image had since been rebuilt as 2026.05.25 after the Rust
toolchain switched to nightly.
This left CI running in an older container where cargo was not installed
for the nightly toolchain selected by the checked-out tree, causing Rust
crate builds to fail during the fuzzer host-tools build.
Update the workflow image references to the current published CI image.
This means running the build inside a Docker container based on a (more)
stable build image.
* The setup action is now largely simplified for Linux.
* We hardcode llvm@21 for macOS since it allows us to get rid of
specifying llvm_version as an input variable. Since we no longer
support switching the LLVM version on Linux via that variable, this
seems like the best course of action.
We use a trick to optionally introduce a container image: `fromJSON()`
allows `null` as a return value, which causes the job to execute on the
runner directly. We need this for macOS and Windows jobs.
These were originally introduced to make it possible to discern between
GitHub's and Blacksmith's caches. This is no longer necessary since
Blacksmith resolved some issues _and_ switched to transparently
proxying GitHub cache actions to their own caches.
