PGFPlotsX: fix log scale power with custom ticks

[t-bltg]

PGFPlotsX fix for #3959.

[codecov]

Codecov Report

Merging #3961 (d7e8ce8) into master (c94cf18) will increase coverage by 0.02%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master    #3961      +/-   ##
==========================================
+ Coverage   62.45%   62.48%   +0.02%     
==========================================
  Files          29       29              
  Lines        7223     7226       +3     
==========================================
+ Hits         4511     4515       +4     
+ Misses       2712     2711       -1     

Impacted Files	Coverage Δ
src/backends/pgfplotsx.jl	60.06% <42.85%> (-0.13%)	⬇️
src/args.jl	71.71% <0.00%> (-0.31%)	⬇️
src/backends/gr.jl	88.40% <0.00%> (+0.41%)	⬆️

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