Fix CI PATH: prepend GNU sed via GITHUB_PATH instead of replacing PATH

The hardcoded job-level PATH env wiped out nvm/node, breaking
actions/checkout. Use GITHUB_PATH to prepend GNU sed's gnubin
directory while preserving the runner's inherited PATH.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitea/workflows/build.yaml

@@ -20,8 +20,6 @@ jobs:
   build:
     runs-on: [self-hosted, macos]
     timeout-minutes: 180
-    env:
-      PATH: /opt/homebrew/opt/gnu-sed/libexec/gnubin:/opt/homebrew/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin
 
     steps:
       - name: Checkout
@@ -35,6 +33,7 @@ jobs:
           for pkg in make gnu-sed crane; do
             brew list --formula "$pkg" &>/dev/null || brew install "$pkg"
           done
+          echo "/opt/homebrew/opt/gnu-sed/libexec/gnubin" >> "$GITHUB_PATH"
           gmake --version | head -1
 
       - name: Set up Docker Buildx

.gitea/workflows/check-updates.yaml

@@ -15,8 +15,6 @@ jobs:
   check-and-build:
     runs-on: [self-hosted, macos]
     timeout-minutes: 15
-    env:
-      PATH: /opt/homebrew/opt/gnu-sed/libexec/gnubin:/opt/homebrew/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin
 
     steps:
       - name: Checkout
@@ -29,6 +27,7 @@ jobs:
           for pkg in make gnu-sed crane jq; do
             brew list --formula "$pkg" &>/dev/null || brew install "$pkg"
           done
+          echo "/opt/homebrew/opt/gnu-sed/libexec/gnubin" >> "$GITHUB_PATH"
 
       - name: Check for upstream updates
         id: check