feat: Use environment variables and startup script in agent (#1147)

These values were ignored. Environment variables are applied to
new sessions, and are refreshed on reconnect. This is cool because
a workspace could be updated with new environment variables without
requiring a complete start/stop.

The startup script is only ran once regardless of changes, which
feels like the expected behavior.

cli/gitssh.go

@@ -7,10 +7,11 @@ import (
 	"os/exec"
 	"strings"
 
-	"github.com/coder/coder/cli/cliui"
-	"github.com/coder/coder/codersdk"
 	"github.com/spf13/cobra"
 	"golang.org/x/xerrors"
+
+	"github.com/coder/coder/cli/cliui"
+	"github.com/coder/coder/codersdk"
 )
 
 func gitssh() *cobra.Command {