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 (9d86897) 69.18%.
Report is 13 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #26334      +/-   ##
==========================================
- Coverage   69.18%   69.18%   -0.01%     
==========================================
  Files        1945     1944       -1     
  Lines       75967    75969       +2     
  Branches     8466     8471       +5     
==========================================
  Hits        52556    52556              
+ Misses      21224    21223       -1     
- Partials     2187     2190       +3     

Flag	Coverage Δ
hive	53.68% <66.66%> (+<0.01%)	⬆️
javascript	56.51% <ø> (+<0.01%)	⬆️
mysql	78.08% <100.00%> (-0.01%)	⬇️
postgres	78.17% <100.00%> (-0.01%)	⬇️
presto	53.63% <66.66%> (+<0.01%)	⬆️
python	82.86% <100.00%> (-0.01%)	⬇️
sqlite	76.83% <100.00%> (-0.01%)	⬇️
unit	55.81% <100.00%> (+0.01%)	⬆️

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