Replace user of source command with (#5818)

Per https://stackoverflow.com/questions/13702425/source-command-not-found-in-sh-shell, `.` is more widely supportedthan `source`. For example, `bin/sh` packaged in Ubuntu does not have `source` but supports `.`.

Credits for @RafaelZasas to discovering the issue on Ubuntu.

Fixes #5636

src/functions/python.ts

@@ -16,7 +16,7 @@ export function virtualEnvCmd(cwd: string, venvDir: string): { command: string;
     process.platform === "win32" ? ["Scripts", "activate.bat"] : ["bin", "activate"];
   const venvActivate = path.join(cwd, venvDir, ...activateScriptPath);
   return {
-    command: process.platform === "win32" ? venvActivate : "source",
+    command: process.platform === "win32" ? venvActivate : ".",
     args: [process.platform === "win32" ? "" : venvActivate],
   };
 }