rust-installer/install-template.sh: improve efficiency, step 1. This round replaces repetitive pattern matching in the inner loop of this script using grep (which causes a `fork()` for each test) with built-in pattern matching in the Bourne shell using the `case` / `esac` construct. This in reference to https://github.com/rust-lang/rust/issues/80684 and is a separated-out request from https://github.com/rust-lang/rust-installer/pull/111 which apparently never got any review. The forthcoming planned "step 2" change builds on top of this change, and replaces the inner-loops needless uses of `sed` (which again causes a `fork()` for each instance) with the suffix removal constructs from the Bourne shell. Since this change touches lots of the same lines this change does, that pull request cannot be submitted before this one is accepted. Hopefully this first step is less controversial than the latter change.