install-binary.sh: Use $HELM_BIN in HELM_HOME assignment (#1030)

When a command named 'helm' does not actually exist (for example when
Helm is invoked as 'helm4' or similar), an error like the following may
be printed even if installation then continues to apparently succeed.

   [/path/to]/helm/plugins/4/helm-diff/install-binary.sh: 19: helm: not found
   mkdir: cannot create directory '': No such file or directory

Author: zware

install-binary.sh

@@ -16,7 +16,7 @@ if command -v cygpath >/dev/null 2>&1; then
 fi
 
 [ -z "$HELM_BIN" ] && HELM_BIN=$(command -v helm)
-[ -z "$HELM_HOME" ] && HELM_HOME=$(helm env | grep 'HELM_DATA_HOME' | cut -d '=' -f2 | tr -d '"')
+[ -z "$HELM_HOME" ] && HELM_HOME=$($HELM_BIN env | grep 'HELM_DATA_HOME' | cut -d '=' -f2 | tr -d '"')
 mkdir -p "$HELM_HOME"
 : "${HELM_PLUGIN_DIR:="$HELM_HOME/plugins/helm-diff"}"