fix(go): select the host CLI package in E2E tests

[Pybsama]

Closes #2116.

What this changes

The Go E2E harness now derives exact @github/copilot-<platform>-<arch>
package candidates from ffihost.PrebuildsFolder() instead of globbing every
copilot-* directory and taking the first sorted result.

• macOS and Windows probe only their exact host package;
• Linux probes the detected glibc/musl package first and the sibling libc
  variant second;
• unrelated or foreign-platform copilot-* packages are ignored;
• CLIPath(testing.TB) reports the packages tried and those actually present,
  replacing the duplicated empty-path guards at its call sites; and
• go/test.sh applies the same exact host OS/architecture mapping.

The shared .github/actions/setup-copilot action is intentionally unchanged,
as called out in the Issue.

Tests

Added unit coverage for:

• macOS, Windows, Linux glibc, and Linux musl candidate generation;
• ignoring alphabetically earlier foreign and non-platform packages;
• respecting Linux libc candidate order;
• rejecting a package without an index.js; and
• listing installed packages for failure diagnostics.

Local validation:

• GOTOOLCHAIN=go1.24.0 go test ./internal/e2e/testharness
• go test ./...
• env -u COPILOT_CLI_PATH bash test.sh
• COPILOT_SDK_DEFAULT_CONNECTION=inprocess GOFLAGS=-tags=copilot_inprocess env -u COPILOT_CLI_PATH bash test.sh
• golangci-lint run ./... --timeout=5m (0 issues)
• bash -n go/test.sh
• cross-compiled the testharness tests for Linux, Windows, and macOS on amd64
  and arm64

Both the default and in-process full race-enabled test runs passed.