Use mise to install zig + go in release CI; cut 0.0.4

`mlugg/setup-zig` was chasing mirrors for ~4 minutes on every run
(see v0.0.1 / v0.0.2 logs) and `actions/setup-go` was spending
another ~4 minutes downloading Go before patterm started building.
mise already manages the project's zig pin; adding `go = "1.26.3"`
to `.mise.toml` (matching go.mod) lets `jdx/mise-action@v2` install
both with one cached step. Subsequent runs reuse the mise cache
instead of re-resolving mirror URLs and re-downloading toolchains.

Also adds an `actions/cache@v4` step for `~/.cache/go-build` and
`~/go/pkg/mod` keyed on `go.sum` so `go build` itself doesn't
re-pull modules every tag push.

.mise.toml

@@ -3,6 +3,8 @@
 # libghostty-vt is built from a pinned upstream Ghostty commit; that
 # commit's build.zig.zon pins minimum_zig_version = 0.15.2. We match
 # it here so contributors don't have to puzzle out the version from
-# a deep upstream file.
+# a deep upstream file. The go pin matches go.mod so CI and local
+# builds use the same toolchain.
 [tools]
 zig = "0.15.2"
+go = "1.26.3"