feat(golangci): use gnopls as GOPACKAGESDRIVER when available

[notJoon]

Summary

• Auto-wire gnopls as the go/packages external driver (GOPACKAGESDRIVER) when both gnopls is on PATH and GNOROOT is set, so golangci-lint can analyze .gno packages whose imports include gno.land/....
• Without this wiring the stock Go loader fails to resolve those imports and golangci-lint exits with empty stdout. The previous fix absorbs that as a silent fallback, so environments lacking gnopls or GNOROOT see no behavioral change.
• GOPACKAGESDRIVER accepts a single executable path; write a per-pid shim (#!/bin/sh\nexec <gnopls> resolve "$@" on unix, .cmd variant on Windows) into the OS temp dir on first use and let the OS reclaim it.
• Cache PATH lookup, shim creation, and the os.Environ() snapshot once per process so the addition is a single atomic load on the per-file hot path.

Background

gnopls implements the go/packages external-driver protocol: its gnopls resolve subcommand walks $GNOROOT/gnovm/stdlibs and $GNOROOT/examples/, parses each directory's gnomod.toml, and serves a fully populated package graph back to any go/packages.Load caller (which golangci-lint uses internally). Setting GOPACKAGESDRIVER to the gnopls shim is enough — no golangci-lint fork needed.

Test plan

• go build ./...
• make test — race + shuffle clean
• Bench: go test -bench=BenchmarkRun -benchmem -run=^$ -benchtime=10x -count=3 ./internal/ matches main to within ±0.1% on B/op and allocs/op
• TestGnoDriverEnv — five subtests covering all detection branches via injected lookPath/getenv/ensureShim
• TestWriteGnoplsShim_{Unix,Windows,FailsOnUnwritableDir} — script body, mode bits, error path
• Manual: with gnopls on PATH and GNOROOT exported, run tlin . against a .gno package and confirm golangci-lint issues now surface
• Manual: without gnopls, confirm fallback (no errors, no per-file warnings)

Notes

• Shim is left in os.TempDir() after the process exits — relies on OS-level temp cleanup. Per-pid filename avoids collisions.
• Fallback is silent: callers see nil env from cachedDriverEnv and golangci-lint runs without a custom driver, which the prior fix already handles.