[Fix] nvm_download, nvm_get_latest, install: only select a downloader that exists as an executable

`nvm_has` matches shell functions and aliases,
but downloads now run via `command`, which skips them
- a `curl` shell function with no curl binary on the PATH would select the curl path and fail with exit 127,
instead of falling back to an available wget executable.
The new `nvm_has_executable` helper resolves names the same way `command` does, so downloader selection and execution agree.

test/fast/Unit tests/nvm_get_latest missing curl or wget

@@ -3,7 +3,7 @@
 die () { echo "$@" ; cleanup ; exit 1; }
 
 cleanup() {
-  unset -f nvm_has
+  unset -f nvm_has nvm_has_executable
 }
 
 : nvm.sh
@@ -12,6 +12,7 @@ cleanup() {
 \. ../../common.sh
 
 nvm_has() { return 1 ; }
+nvm_has_executable() { return 1 ; }
 
 try_err nvm_get_latest
 [ "_$CAPTURED_STDERR" = "_nvm needs curl or wget to proceed." ] \