fix: downgrade go-fuse to v2.11.0 and add linux/386 build check

[kgala2]

Description

Downgrades github.com/hanwen/go-fuse/v2 back to v2.11.0 and adds a 32-bit Linux (GOOS=linux GOARCH=386) compilation check in GitHub Actions.

Root Cause

In go-fuse v2.12.0 (commit d177ec7), splice/pair_linux.go explicitly typed variables receiving syscall.Splice return values as int64. In Go's standard library on Linux:

• 64-bit architectures (amd64, arm64): syscall.Splice returns (int64, error)
• 32-bit architectures (386, arm): syscall.Splice returns (int, error)

This broke compilation on 32-bit Linux with:

splice/pair_linux.go:31:14: cannot use syscall.Splice(int(rfd), nil, int(wfd), nil, sz, 0) (value of type int) as int64 value in assignment

which failed the Kokoro continuous and continuous-test release binary builds for cloud-sql-proxy.linux.386.

Upstream issue filed: hanwen/go-fuse#628.

Changes

1. go.mod: Downgrade github.com/hanwen/go-fuse/v2 to v2.11.0.
2. .github/workflows/tests.yaml: Add CGO_ENABLED=0 GOOS=linux GOARCH=386 go build under the compilation check so 32-bit build breakages are detected in PR CI.

Verification

• CGO_ENABLED=0 GOOS=linux GOARCH=386 go build passes locally.
• CGO_ENABLED=0 GOOS=freebsd go build and openbsd pass locally.
• go test -short ./... passes.