ci(nix): retry flake evaluation on GitHub API rate limits

Add a reusable retry helper that detects GitHub API 403 rate-limit errors
during Nix flake evaluation and retries with exponential backoff.

Apply the retry logic to flake-only CI tests so transient GitHub rate
limits no longer cause random CI failures while preserving fast failure
for real errors.

https://chatgpt.com/share/693d7ec5-ac70-800f-a627-ef705c653ba1

scripts/nix/lib/retry_403.sh

@@ -0,0 +1,52 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+if [[ -n "${PKGMGR_NIX_RETRY_403_SH:-}" ]]; then
+  return 0
+fi
+PKGMGR_NIX_RETRY_403_SH=1
+
+# Retry only when we see the GitHub API rate limit 403 error during nix flake evaluation.
+# Retries 7 times with delays: 10, 30, 50, 80, 130, 210, 420 seconds.
+run_with_github_403_retry() {
+  local -a delays=(10 30 50 80 130 210 420)
+  local attempt=0
+  local max_retries="${#delays[@]}"
+
+  while true; do
+    local err tmp
+    tmp="$(mktemp -t nix-err.XXXXXX)"
+    err=0
+
+    # Run the command; capture stderr for inspection while preserving stdout.
+    if "$@" 2>"$tmp"; then
+      rm -f "$tmp"
+      return 0
+    else
+      err=$?
+    fi
+
+    # Only retry on the specific GitHub API rate limit 403 case.
+    if grep -qE 'HTTP error 403' "$tmp" && grep -qiE 'API rate limit exceeded|api\.github\.com' "$tmp"; then
+      if (( attempt >= max_retries )); then
+        cat "$tmp" >&2
+        rm -f "$tmp"
+        return "$err"
+      fi
+
+      local sleep_s="${delays[$attempt]}"
+      attempt=$((attempt + 1))
+
+      echo "[nix-retry] GitHub API rate-limit (403). Retry ${attempt}/${max_retries} in ${sleep_s}s: $*" >&2
+      cat "$tmp" >&2
+      rm -f "$tmp"
+      sleep "$sleep_s"
+      continue
+    fi
+
+    # Not our retry case -> fail fast with original stderr.
+    cat "$tmp" >&2
+    rm -f "$tmp"
+    return "$err"
+  done
+}

scripts/test/test-env-nix.sh

@@ -27,7 +27,7 @@ docker run --rm \
     echo ">>> preflight: nix must exist in image"
     if ! command -v nix >/dev/null 2>&1; then
       echo "NO_NIX"
-      echo "ERROR: nix not found in image '\'''"${IMAGE}"''\'' (PKGMGR_DISTRO='"${PKGMGR_DISTRO}"')"
+      echo "ERROR: nix not found in image '"${IMAGE}"' (PKGMGR_DISTRO='"${PKGMGR_DISTRO}"')"
       echo "HINT: Ensure Nix is installed during image build for this distro."
       exit 1
     fi
@@ -35,14 +35,28 @@ docker run --rm \
     echo ">>> nix version"
     nix --version
 
+    # ------------------------------------------------------------
+    # Retry helper for GitHub API rate-limit (HTTP 403)
+    # ------------------------------------------------------------
+    if [[ -f /src/scripts/nix/lib/retry_403.sh ]]; then
+      # shellcheck source=./scripts/nix/lib/retry_403.sh
+      source /src/scripts/nix/lib/retry_403.sh
+    elif [[ -f ./scripts/nix/lib/retry_403.sh ]]; then
+      # shellcheck source=./scripts/nix/lib/retry_403.sh
+      source ./scripts/nix/lib/retry_403.sh
+    else
+      echo "ERROR: retry helper not found: scripts/nix/lib/retry_403.sh"
+      exit 1
+    fi
+
     echo ">>> nix flake show"
-    nix flake show . --no-write-lock-file >/dev/null
+    run_with_github_403_retry nix flake show . --no-write-lock-file >/dev/null
 
     echo ">>> nix build .#default"
-    nix build .#default --no-link --no-write-lock-file
+    run_with_github_403_retry nix build .#default --no-link --no-write-lock-file
 
     echo ">>> nix run .#pkgmgr -- --help"
-    nix run .#pkgmgr -- --help --no-write-lock-file
+    run_with_github_403_retry nix run .#pkgmgr -- --help --no-write-lock-file
 
     echo ">>> OK: Nix flake-only test succeeded."
   '