Rebuild TZData before benchmarks

[Wynand]

This recompiles TZData before running benchmarks to ensure benchmarks still run when the serialized TZdata is changed

[Wynand]

Here's how I tested this:

My script for running the benchmarks was

git clean -dfX && julia --project=@. -e 'using Pkg; Pkg.build("TimeZones")'
git fetch origin +:refs/remotes/origin/HEAD
julia --project=benchmark/ -e 'using Pkg; Pkg.instantiate(); Pkg.develop(PackageSpec(path=pwd()))'
julia --project=benchmark/ -e 'using PkgBenchmark, TimeZones; export_markdown(stdout, judge(TimeZones, "origin/HEAD", verbose=false))'

And then when comparing to changes in #354 I added the same changes and ran:

git clean -dfX && julia --project=@. -e 'using Pkg; Pkg.build("TimeZones")'
git fetch origin +:refs/remotes/origin/HEAD
julia --project=benchmark/ -e 'using Pkg; Pkg.instantiate(); Pkg.develop(PackageSpec(path=pwd()))'
julia --project=benchmark/ -e 'using PkgBenchmark, TimeZones; export_markdown(stdout, judge(TimeZones, "wy/clean-benchmark", verbose=false))'