Fix system image incompatibility with LibCURL.cacert

[omus]

When attempting to reference LibCURL.cacert from within a system image the returned path would be "" as this is what MozillaCACerts_jll stores before initialization.

[codecov-commenter]

Codecov Report

Merging #84 into master will increase coverage by 0.70%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##           master     #84      +/-   ##
=========================================
+ Coverage    6.81%   7.51%   +0.70%     
=========================================
  Files           2       2              
  Lines         132     133       +1     
=========================================
+ Hits            9      10       +1     
  Misses        123     123              

Impacted Files	Coverage Δ
src/LibCURL.jl	41.66% <100.00%> (+5.30%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 87ca1d1...104fae2. Read the comment docs.

[omus]

I was looking into writing some tests for this but I've been unable to reproduce the failure that existed without this change using something like:

using PackageCompiler
create_sysimage(:LibCURL; sysimage_path="LibCURL.so")
run(`$(Base.julia_cmd()) -JLibCURL.so -E '@assert !isempty(LibCURL.cacert)'`)

I may look into this further but I don't think it's essential to add a test here if I can confirm this fixes the issue.

[iamed2]

Globals should be set as const Ref values rather than global

[omus]

Globals should be set as const Ref values rather than global

I don't disagree. However, I want this change to be non-breaking and using a Ref requires this to be a breaking change then. Also since this is just a copy of what MozillaCACerts_jll is doing probably an issue should be created there.

[omus]

I've made an issue against BinaryBuilder.jl: JuliaPackaging/BinaryBuilder.jl#829

[omus]

I have confirmed that this fixes the incompatibility with the system image.