ci: add retry logic for Go module operations (#21609)

## Description

Add exponential backoff retries to all `go install` and `go mod
download` commands across CI workflows and actions.

## Why

Fixes
[coder/internal#1276](https://github.com/coder/internal/issues/1276) -
CI fails when `sum.golang.org` returns 500 errors during Go module
verification. This is an infrastructure-level flake that can't be
controlled.

## Changes

- Created `.github/scripts/retry.sh` - reusable retry helper with
exponential backoff (2s, 4s, 8s delays, max 3 attempts), using
`scripts/lib.sh` helpers
- Wrapped all `go install` and `go mod download` commands with retry in:
  - `.github/actions/setup-go/action.yaml`
  - `.github/actions/setup-sqlc/action.yaml`
  - `.github/actions/setup-go-tools/action.yaml`
  - `.github/workflows/ci.yaml`
  - `.github/workflows/release.yaml`
  - `.github/workflows/security.yaml`
- Added GNU tools setup (bash 4+, GNU getopt, make 4+) for macOS in
`test-go-pg` job, since `retry.sh` uses `lib.sh` which requires these
tools

.github/actions/setup-sqlc/action.yaml

@@ -14,4 +14,4 @@ runs:
       # - https://github.com/sqlc-dev/sqlc/pull/4159
       shell: bash
       run: |
-        CGO_ENABLED=1 go install github.com/coder/sqlc/cmd/sqlc@aab4e865a51df0c43e1839f81a9d349b41d14f05
+        ./.github/scripts/retry.sh -- env CGO_ENABLED=1 go install github.com/coder/sqlc/cmd/sqlc@aab4e865a51df0c43e1839f81a9d349b41d14f05