fix: stringify scarf pixel value

[eschutho]

SUMMARY

In order to match the expression here: https://github.com/apache/superset/blob/master/superset-frontend/src/components/TelemetryPixel/index.tsx#L48
we need to cast the env var for SCARF_ANALYTICS to a string in the webpack config.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (04f1c35) 69.18% compared to head (34c15c9) 69.18%.
Report is 35 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #26351   +/-   ##
=======================================
  Coverage   69.18%   69.18%           
=======================================
  Files        1945     1945           
  Lines       75967    75982   +15     
  Branches     8466     8475    +9     
=======================================
+ Hits        52556    52568   +12     
  Misses      21224    21224           
- Partials     2187     2190    +3     

Flag	Coverage Δ
javascript	56.52% <ø> (+<0.01%)	⬆️

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[rusackas]

I fear this might cause as much ambiguity as it solves, but I'll do some testing to confirm...