gnu: rust: Work around rust 1.25...1.27 reproducibility issues.

gnu: rust: Work around rust 1.25...1.27 reproducibility issues.

* gnu/packages/patches/rust-mdbook-support-reproducible-builds-by-forcing-window.search.patch:
New file.
* gnu/local.mk (dist_patch_DATA): Add new patch file.
* gnu/packages/rust.scm (rust-1.19): Use system libssh2 library.
during cargo build.  Note: libgit2 still bundled, because cargo
tests assume specific libgit2 minor release.
(rust-1.23): Inherit native-inputs from previous package.
(rust-1.25): Switch back to llvm 3.9.1 as workaround for
https://github.com/rust-lang/rust/issues/50556 issue.
(rust-1.27): Apply changes from
https://github.com/rust-lang-nursery/mdBook/pull/692 to make
generation of "searchindex.js" files reproducible. Disable cargo
test that required llvm 6.