Build mbedTLS, libssh2, nghttp2 and curl disregarding `USE_SYSTEM_LIBGIT2=1` (#54367)
A long time ago, these libraries were only used indirectly by libgit2,
so
https://github.com/JuliaLang/julia/pull/18153/commits/4526b65d7bfaf75e904f77b03e33f554d7ab5d14
made building them conditional on `USE_SYSTEM_LIBGIT2=0`. But nowadays
we always generate MbedTLS_jll, LibSSH2_jll, nghttp2_jll and
LibCURL_jll, even when not building libgit2, so we always need to have
these libraries available.