use `userprofile` environment variable in windows `keytool` command

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Description of what this PR is changing or adding, and why:

the userprofile variable is defined in the registry at HKEY_CURRENT_USER\Volatile Environment. this change allows the user to copy and paste the codeblock easily without having to modify it

Presubmit checklist

• [X] This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• [X] This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• [X] This PR uses semantic line breaks of 80 characters or fewer.