Fix suggestios done in code review

Rollback a not abot advanced usage on how to use the file and not use
the GITHUB_WORKSPACE since it was removed in a previous PR

Author: elpic

docs/advanced-usage.md

@@ -242,7 +242,7 @@ jobs:
 The python-version-file input accepts a path to a file containing the version of Python to be used by a project, for example .python-version, or .tool-versions. 
 If both the python-version and the python-version-file inputs are provided then the python-version input is used. 
 
-> The action will search for the python version file relative to the repository root.
+> In case both `python-version` and `python-version-file` inputs are supplied, the `python-version-file` input will be ignored due to its lower priority.
 
 ```yaml
 steps:

src/setup-python.ts

@@ -37,18 +37,13 @@ function resolveVersionInput() {
   }
 
   if (versionFile) {
-    const versionFilePath = path.join(
-      process.env.GITHUB_WORKSPACE!,
-      versionFile
-    );
-
-    if (!fs.existsSync(versionFilePath)) {
+    if (!fs.existsSync(versionFile)) {
       throw new Error(
-        `The specified python version file at: ${versionFilePath} doesn't exist.`
+        `The specified python version file at: ${versionFile} doesn't exist.`
       );
     }
 
-    const version = parsePythonVersionFile(fs.readFileSync(versionFilePath, 'utf8'));
+    const version = parsePythonVersionFile(fs.readFileSync(versionFile, 'utf8'));
     core.info(`Resolved ${versionFile} as ${version}`);
 
     return [version];