feat: use self-update to modify version if mise is already installed (#277)

Hello 👋 

1st of all thanks for your work on mise, it's really a game changer 😁 

## Why
This PR tries to address the issue #165 (mise-action cannot upgrade
mise)

## What
I've decided to use `mise self-update` if mise is already installed to
switch to the desired version
Let me know if this is an OK fix for you

## Tests
To test this feature, I've run the action on my repo and screenshot the
newly created output

<img width="591" height="538" alt="Screenshot 2025-10-01 at 22 14 46"
src="https://github.com/user-attachments/assets/9a10a33a-d86f-410b-8bdc-71b07b3036f6"
/>

The sha of the commit was suposed to match the sha of the screenshot,
but I've fixup and rebase the PR
Happy to do a new one if you want live proof

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> When mise is already installed, compare the requested `version` input
to the installed version and run `self-update` to align if they differ.
> 
> - **Action Logic (`src/index.ts`)**:
>   - **Existing Installation Handling**:
> - Read `version` input; fetch installed version via `mise version
--json`.
> - If versions differ, execute `mise self-update <version> -y`;
otherwise log that mise is already installed.
>   - Download path and SHA256 verification remain unchanged.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
941208cb9b7a6e796cdc7e96a8dcf6560d4d207c. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

dist/index.js

@@ -50194,6 +50194,24 @@ async function setupMise(version, fetchFromGitHub = false) {
                 break;
         }
     }
+    else {
+        const requestedVersion = core.getInput('version');
+        if (requestedVersion !== '') {
+            const versionOutput = await exec.getExecOutput(miseBinPath, ['version', '--json'], { silent: true });
+            const versionJson = JSON.parse(versionOutput.stdout);
+            const version = versionJson.version.split(' ')[0];
+            if (requestedVersion === version) {
+                core.info(`mise already installed`);
+            }
+            else {
+                core.info(`mise already installed (${version}), but different version requested (${requestedVersion})`);
+                await exec.exec(miseBinPath, ['self-update', requestedVersion, '-y'], {
+                    silent: true
+                });
+                core.info(`mise updated to version ${requestedVersion}`);
+            }
+        }
+    }
     // compare with provided hash
     const want = core.getInput('sha256');
     if (want) {

src/index.ts

@@ -264,6 +264,28 @@ async function setupMise(
         await exec.exec('chmod', ['+x', miseBinPath])
         break
     }
+  } else {
+    const requestedVersion = core.getInput('version')
+    if (requestedVersion !== '') {
+      const versionOutput = await exec.getExecOutput(
+        miseBinPath,
+        ['version', '--json'],
+        { silent: true }
+      )
+      const versionJson = JSON.parse(versionOutput.stdout)
+      const version = versionJson.version.split(' ')[0]
+      if (requestedVersion === version) {
+        core.info(`mise already installed`)
+      } else {
+        core.info(
+          `mise already installed (${version}), but different version requested (${requestedVersion})`
+        )
+        await exec.exec(miseBinPath, ['self-update', requestedVersion, '-y'], {
+          silent: true
+        })
+        core.info(`mise updated to version ${requestedVersion}`)
+      }
+    }
   }
   // compare with provided hash
   const want = core.getInput('sha256')