fix spurious GR warning

[t-bltg]

Per #4584 (comment).

Fix a spurious warning when GR is in the Manifest, but not in the Project.

[BeastyBlacksmith]

To test this I did the following:

JULIA_DEPOT_PATH=$(mktemp -d) julia

pkg> add Plots
# Ctrl-C when precompilation started
julia> using Plots

[BeastyBlacksmith]

Need to fix CI though ^^

[t-bltg]

The warning also appeared in PlotUtils (https://github.com/JuliaPlots/PlotUtils.jl/actions/runs/4015693175/jobs/6897730588#step:6:387), so I could find a fix.

CI failure is odd, will have a look into that.

[t-bltg]

That's all related to GR being a hard Plots dependency.

This warning would not have occurred if #4631 was fixed (but we cannot do that until Plots@2.0).

[codecov]

Codecov Report

Base: 90.80% // Head: 90.80% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (b0df12f) compared to base (54a215f).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4641   +/-   ##
=======================================
  Coverage   90.80%   90.80%           
=======================================
  Files          41       41           
  Lines        8778     8781    +3     
=======================================
+ Hits         7971     7974    +3     
  Misses        807      807           

Impacted Files	Coverage Δ
src/backends.jl	82.58% <100.00%> (+0.26%)	⬆️

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