--platforms silently ignores multiple entries

### Description of the bug:

`--platforms` only use a single label. If multiple labels are passed, all but the first are dropped silently, **without** any kind of warning or error message.

### Which category does this issue belong to?

_No response_

### What's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

`bazel build //:all --platforms=@platforms//host,this-extra-platform-is-ignored-without-warning-or-error`

### Which operating system are you running Bazel on?

Linux

### What is the output of `bazel info release`?

release 9.2.0

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse HEAD` ?

```text

```

### If this is a regression, please try to identify the Bazel commit where the bug was introduced with bazelisk --bisect.

Commit 81acd2a352387399914ea1ef2a5efa1b4ecc766b effectively removed the [printing of a warning message](https://github.com/bazelbuild/bazel/blob/f644f2dff90cbbba0e14551051578c5e93328650/src/main/java/com/google/devtools/build/lib/analysis/PlatformConfiguration.java#L55-L62).

### Have you found anything relevant by searching the web?

_No response_

### Any other information, logs, or outputs that you want to share?

_No response_

Start in src/main/java/com/google/devtools/build/lib/analysis/PlatformConfiguration.java and inspect commit 81acd2a352387399914ea1ef2a5efa1b4ecc766b against the earlier warning logic. Reproduce the behavior with the provided bazel build command; done means multiple --platforms entries are no longer silently dropped and the user receives the intended warning or error.

Written by the indexing model from the issue text.