Support using minimum_zig_version from build.zig.zon

This is a breaking change since `version` being empty now has a different
meaning from before.

Resolves: #16

Author: github-actions[bot]

action.yml

@@ -2,9 +2,8 @@ name: 'Setup Zig Compiler'
 description: 'Download and install the Zig compiler, and cache the global Zig cache'
 inputs:
   version:
-    description: 'Version of the Zig compiler, e.g. "0.13.0" or "0.13.0-dev.351+64ef45eb0". "master" uses the latest nightly build. "latest" uses the latest tagged release.'
-    required: true
-    default: 'latest'
+    description: 'Version of the Zig compiler, e.g. "0.13.0" or "0.13.0-dev.351+64ef45eb0". "master" uses the latest nightly build. "latest" uses the latest tagged release. Leave empty to use minimum_zig_version from build.zig.zon, with a fallback to "latest".'
+    default: ''
   mirror:
     description: 'Override of Zig download mirror to use, e.g. "https://pkg.machengine.org/zig".'
     required: false