Add System.Reflection.Metadata as a dependency for netcoreapp3.0 target

[Tyrrrz]

Fixes #1050

[codecov-commenter]

Codecov Report

Merging #1064 (7a00e77) into main (97ef30f) will decrease coverage by 1.60%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1064      +/-   ##
==========================================
- Coverage   81.16%   79.55%   -1.61%     
==========================================
  Files         194      194              
  Lines        6349     6349              
  Branches     1530     1530              
==========================================
- Hits         5153     5051     -102     
- Misses        758      868     +110     
+ Partials      438      430       -8     

Impacted Files	Coverage Δ
src/Sentry/PlatformAbstractions/FrameworkInfo.cs	0.00% <0.00%> (-100.00%)	⬇️
...ntry/PlatformAbstractions/RegistryKeyExtensions.cs	0.00% <0.00%> (-100.00%)	⬇️
...Sentry/PlatformAbstractions/FrameworkInfo.NetFx.cs	0.00% <0.00%> (-71.24%)	⬇️
...rmAbstractions/NetFxInstallationsEventProcessor.cs	4.54% <0.00%> (-68.19%)	⬇️
...ntry/Integrations/NetFxInstallationsIntegration.cs	33.33% <0.00%> (-66.67%)	⬇️
...ntry/PlatformAbstractions/FrameworkInstallation.cs	25.00% <0.00%> (-37.50%)	⬇️
src/Sentry/PlatformAbstractions/RuntimeInfo.cs	53.44% <0.00%> (-5.18%)	⬇️
src/Sentry/GlobalSessionManager.cs	63.15% <0.00%> (-1.76%)	⬇️
src/Sentry/Internal/AppDomainAdapter.cs	66.66% <0.00%> (ø)
src/Sentry/Internal/BackgroundWorker.cs	84.93% <0.00%> (+0.68%)	⬆️

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