Apt Signing Key has "None" Content Type

### Description of the bug:

The content type of the `https://bazel.build/bazel-release.pub.gpg` key is `None`, even when a key is successfully returned. This key is referenced in the https://bazel.build/install/ubuntu setup instructions.

This differs from other GPG key providers. For example: `curl https://apt.llvm.org/llvm-snapshot.gpg.key` returns a key with the content type `application/octet-stream`.

I would expect the `bazel-release.pub.gpg` key to also have a content type of `application/octet-stream` or an otherwise helpful content type.

This unexpected `None` type makes it impossible for Sonatype's Nexus (and perhaps other automation that parses the content type) to proxy this key.

### Which category does this issue belong to?

Local Execution

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

Run:
```bash
$ curl https://bazel.build/bazel-release.pub.gpg -v 2>&1 | grep -i '< content-type:'
```
Which should return:
```text
< content-type: None
```

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

_No response_

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

_No response_

### 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` ?

_No response_

### Is this a regression? If yes, please try to identify the Bazel commit where the bug was introduced.

_No response_

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

_No response_

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

The Bazel GPG key returning a `None` content type:
```bash
$ curl https://bazel.build/bazel-release.pub.gpg -v 2>&1 | grep -i '< content-type:'
< content-type: None
```

LLVM key returning `application/octet-stream` content type:
```bash
$ curl https://apt.llvm.org/llvm-snapshot.gpg.key -v 2>&1 | grep -i '< content-type:'
< content-type: application/octet-stream
```