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-go/action.yaml

@@ -22,14 +22,14 @@ runs:
 
     - name: Install gotestsum
       shell: bash
-      run: go install gotest.tools/gotestsum@0d9599e513d70e5792bb9334869f82f6e8b53d4d # main as of 2025-05-15
+      run: ./.github/scripts/retry.sh -- go install gotest.tools/gotestsum@0d9599e513d70e5792bb9334869f82f6e8b53d4d # main as of 2025-05-15
 
     - name: Install mtimehash
       shell: bash
-      run: go install github.com/slsyy/mtimehash/cmd/mtimehash@a6b5da4ed2c4a40e7b805534b004e9fde7b53ce0 # v1.0.0
+      run: ./.github/scripts/retry.sh -- go install github.com/slsyy/mtimehash/cmd/mtimehash@a6b5da4ed2c4a40e7b805534b004e9fde7b53ce0 # v1.0.0
 
     # It isn't necessary that we ever do this, but it helps
     # separate the "setup" from the "run" times.
     - name: go mod download
       shell: bash
-      run: go mod download -x
+      run: ./.github/scripts/retry.sh -- go mod download -x