ci: only accept a numeric nightly major

[ostapondo]

Follow-up to #5654.

The guard I landed there only catches an empty string and the literal
"null". Anything else the download index hands back ends up in
needs.nightly-version.outputs.major, which nodejs-shared.yml drops into a
github-script block. Checking for digits is what it was meant to do.

Ran the check by hand: 27 passes; empty, "null", "8abc" and a string with
quotes in it all fail.

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Review ready

[mcollina]

lgtm

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.44%. Comparing base (6aef2fb) to head (9bc4513).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5657      +/-   ##
==========================================
+ Coverage   93.33%   93.44%   +0.10%     
==========================================
  Files         110      110              
  Lines       38723    38723              
==========================================
+ Hits        36143    36184      +41     
+ Misses       2580     2539      -41     

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