Use NetworkOptions.ca_roots_path()

[mkitti]

Currently, the location of the certificate roots file is hard coded:

Git.jl/src/git_function.jl

Line 33 in a7fd3e9

ssl_cert = joinpath(dirname(Sys.BINDIR), "share", "julia", "cert.pem")

However, this should be determined by NetworkOptions.jl:
https://github.com/JuliaLang/NetworkOptions.jl#ca_roots_path

One reason for this is that the location of the certificate roots file should be able to be overriden by changing JULIA_SSL_CA_ROOTS_PATH.