fix: Start login shells on macOS and Linux (#2437)

This appends `-l` to the shell command on macOS and Linux.
It also adds environment variable expansion to allow for
chaining from `coder_agent.env`.

cli/agent.go

@@ -2,6 +2,7 @@ package cli
 
 import (
 	"context"
+	"fmt"
 	"net/http"
 	_ "net/http/pprof" //nolint: gosec
 	"net/url"
@@ -138,6 +139,15 @@ func workspaceAgent() *cobra.Command {
 				}
 			}
 
+			executablePath, err := os.Executable()
+			if err != nil {
+				return xerrors.Errorf("getting os executable: %w", err)
+			}
+			err = os.Setenv("PATH", fmt.Sprintf("%s%c%s", os.Getenv("PATH"), filepath.ListSeparator, filepath.Dir(executablePath)))
+			if err != nil {
+				return xerrors.Errorf("add executable to $PATH: %w", err)
+			}
+
 			closer := agent.New(client.ListenWorkspaceAgent, &agent.Options{
 				Logger: logger,
 				EnvironmentVariables: map[string]string{