docs(readme): update nightly badge to include pre-releases

As of build-emacs-for-macos v0.6.8, the emacs-builder tool supports
handling stable Emacs release differently, so we can easily separate
them from the regular nightly builds.

.github/workflows/build.yml

@@ -40,7 +40,7 @@ jobs:
         uses: actions/checkout@v2
         with:
           repository: jimeh/build-emacs-for-macos
-          ref: "v0.6.6"
+          ref: "v0.6.8"
           path: builder
       - uses: actions/setup-go@v2
         with:
@@ -96,7 +96,7 @@ jobs:
         uses: actions/checkout@v2
         with:
           repository: jimeh/build-emacs-for-macos
-          ref: "v0.6.6"
+          ref: "v0.6.8"
           path: builder
       - uses: ruby/setup-ruby@v1
         with:

README.md

@@ -8,7 +8,7 @@
 
 <p align="center">
   <a href="https://github.com/jimeh/emacs-builds/releases/latest">
-    <img alt="GitHub release (latest)" src="https://img.shields.io/github/v/release/jimeh/emacs-builds?style=flat&label=nightly&color=%237F5AB6&logo=GNU%20Emacs&logoColor=white">
+    <img alt="GitHub release (latest)" src="https://img.shields.io/github/v/release/jimeh/emacs-builds?include_prereleases&style=flat&label=nightly&color=%237F5AB6&logo=GNU%20Emacs&logoColor=white&sort=semver">
   </a>
   <a href="https://github.com/jimeh/emacs-builds/issues">
     <img alt="GitHub issues" src="https://img.shields.io/github/issues-raw/jimeh/emacs-builds?style=flat&logo=github&colorColor=white">