chore: check for valid id in acceptFolder

Also make sure the LinkedJdkProvider does not use `acceptFolder` when
getting the list of available JDKs.

Author: quintesse

src/main/java/dev/jbang/devkitman/jdkproviders/BaseFoldersJdkProvider.java

@@ -134,7 +134,7 @@ protected String jdkId(String name) {
 	}
 
 	protected boolean acceptFolder(@NonNull Path jdkFolder) {
-		return JavaUtils.hasJavacCmd(jdkFolder);
+		return isValidId(jdkFolder.getFileName().toString()) && JavaUtils.hasJavacCmd(jdkFolder);
 	}
 
 	@Override

src/main/java/dev/jbang/devkitman/jdkproviders/LinkedJdkProvider.java

@@ -50,7 +50,7 @@ public boolean canUse() {
 		String[] parts = idOrToken.split("@", 2);
 		if (parts.length == 2 && isValidId(parts[0]) && isValidPath(parts[1])) {
 			Path jdkPath = Paths.get(parts[1]);
-			if (super.acceptFolder(jdkPath)) {
+			if (JavaUtils.hasJavacCmd(jdkPath)) {
 				Optional<String> version = JavaUtils.resolveJavaVersionStringFromPath(jdkPath);
 				if (!version.isPresent()) {
 					throw new IllegalArgumentException(