fix: exec command on Astro new version message (#10991)

`getExecCommand` is an async function but it wasn't awaited in
`newVersionAvailable` so the message was:
`Run [object Promise] @astrojs/upgrade to update`

packages/astro/src/core/dev/dev.ts

@@ -67,7 +67,7 @@ export default async function dev(inlineConfig: AstroInlineConfig): Promise<DevS
 
 						logger.warn(
 							'SKIP_FORMAT',
-							msg.newVersionAvailable({
+							await msg.newVersionAvailable({
 								latestVersion: version,
 							})
 						);

packages/astro/src/core/messages.ts

@@ -105,10 +105,10 @@ export function serverShortcuts({ key, label }: { key: string; label: string }):
 	return [dim('  Press'), key, dim('to'), label].join(' ');
 }
 
-export function newVersionAvailable({ latestVersion }: { latestVersion: string }) {
+export async function newVersionAvailable({ latestVersion }: { latestVersion: string }) {
 	const badge = bgYellow(black(` update `));
 	const headline = yellow(`▶ New version of Astro available: ${latestVersion}`);
-	const execCommand = getExecCommand();
+	const execCommand = await getExecCommand();
 
 	const details = `  Run ${cyan(`${execCommand} @astrojs/upgrade`)} to update`;
 	return ['', `${badge} ${headline}`, details, ''].join('\n');