bootstap: create .cargo/config only if not present
bootstap: create .cargo/config only if not present

In some situations we should want on influence into the .cargo/config
when we use vendored source.  One example is #90764, when we want to
workaround some references to crates forked and living in git, that are
missing in the vendor/ directory.

This commit will create the .cargo/config file only when the .cargo/
directory needs to be created.

graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust