Fix venv activation for cshell (#22852)

Closes #22822

Use current shell to figure out whether shell integration is working,
even when using fallback shell for getting environment variables.

src/client/terminals/envCollectionActivation/service.ts

@@ -188,7 +188,7 @@ export class TerminalEnvVarCollectionService implements IExtensionActivationServ
 
         // PS1 in some cases is a shell variable (not an env variable) so "env" might not contain it, calculate it in that case.
         env.PS1 = await this.getPS1(shell, resource, env);
-        const prependOptions = await this.getPrependOptions(shell);
+        const prependOptions = await this.getPrependOptions();
 
         // Clear any previously set env vars from collection
         envVarCollection.clear();
@@ -345,8 +345,8 @@ export class TerminalEnvVarCollectionService implements IExtensionActivationServ
         }
     }
 
-    private async getPrependOptions(shell: string): Promise<EnvironmentVariableMutatorOptions> {
-        const isActive = await this.shellIntegrationService.isWorking(shell);
+    private async getPrependOptions(): Promise<EnvironmentVariableMutatorOptions> {
+        const isActive = await this.shellIntegrationService.isWorking(this.applicationEnvironment.shell);
         // Ideally we would want to prepend exactly once, either at shell integration or process creation.
         // TODO: Stop prepending altogether once https://github.com/microsoft/vscode/issues/145234 is available.
         return isActive