Check whether the version in flutter-candidate.txt is a tag or a commit hash (#7993)

Follow-up to https://github.com/flutter/devtools/pull/7989

Fixes error running `devtools_tool update-flutter-sdk` when the Flutter version is a commit hash:

```
~/develop/devtools/packages/devtools_app/ [ext-settings-button*] devtools_tool update-flutter-sdk --use-cache
Using Flutter SDK from /Users/kenzieschmoll/develop/devtools/tool/flutter-sdk
Updating to Flutter version from cache: tags/1c77696615558bf8f9fa7a6e2a473f438f1c03ca 
Updating Flutter at /Users/kenzieschmoll/develop/devtools/tool/flutter-sdk
/Users/kenzieschmoll/develop/devtools/tool/flutter-sdk > git fetch
/Users/kenzieschmoll/develop/devtools/tool/flutter-sdk > git checkout tags/1c77696615558bf8f9fa7a6e2a473f438f1c03ca -f
error: pathspec 'tags/1c77696615558bf8f9fa7a6e2a473f438f1c03ca' did not match any file(s) known to git
ProcessException: Failed with exit code: 1. 
  Command: git checkout tags/1c77696615558bf8f9fa7a6e2a473f438f1c03ca -f
```