pgfplotsx: fix tex labels already in mathmode form

[t-bltg]

Fix #4308.

[codecov]

Codecov Report

Merging #4310 (5a6d5bb) into master (ec9b7e8) will increase coverage by 0.23%.
The diff coverage is 91.66%.

❗ Current head 5a6d5bb differs from pull request most recent head bb881cd. Consider uploading reports for the commit bb881cd to get more accurate results

@@            Coverage Diff             @@
##           master    #4310      +/-   ##
==========================================
+ Coverage   79.98%   80.21%   +0.23%     
==========================================
  Files          29       29              
  Lines        7004     7007       +3     
==========================================
+ Hits         5602     5621      +19     
+ Misses       1402     1386      -16     

Impacted Files	Coverage Δ
src/backends/pgfplotsx.jl	75.40% <91.66%> (+2.31%)	⬆️
src/utils.jl	75.32% <0.00%> (+0.65%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[t-bltg]

@BioTurboNick, you can release 1.31.7 if you wish.