Ensure sharp.format.heif is AVIF-only for prebuilt binaries

lib/utility.js

@@ -75,6 +75,13 @@ if (!libvipsVersion.isGlobal) {
 }
 versions.sharp = require('../package.json').version;
 
+/* istanbul ignore next */
+if (versions.heif && format.heif) {
+  // Prebuilt binaries provide AV1
+  format.heif.input.fileSuffix = ['.avif'];
+  format.heif.output.alias = ['avif'];
+}
+
 /**
  * Gets or, when options are provided, sets the limits of _libvips'_ operation cache.
  * Existing entries in the cache will be trimmed after any change in limits.