[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_download wget Authorization header

@@ -6,7 +6,7 @@ TEST_BIN="$WORK/bin"
 ARGV_LOG="$WORK/argv.log"
 
 cleanup() {
-  unset -f die cleanup nvm_has
+  unset -f die cleanup nvm_has_executable
   rm -rf "$WORK"
   export PATH="$OLDPATH"
 }
@@ -30,7 +30,7 @@ chmod +x "$TEST_BIN/wget"
 export ARGV_LOG
 export PATH="$TEST_BIN:$OLDPATH"
 # force the wget path while keeping system tools (sed) available for sanitization
-nvm_has() { [ "$1" != curl ] && command -v "$1" >/dev/null 2>&1; }
+nvm_has_executable() { [ "$1" != curl ] && command -v "$1" >/dev/null 2>&1; }
 
 # given an Authorization credential in NVM_AUTH_HEADER
 # when nvm_download uses the wget path