packetbeat/beater: help the GC clean up the Npcap installer if it's not used

[efd6]

What does this PR do?

The installer bytes are released in the install case but this was accidentally removed in #30048. This frees ~1MB of runtime memory after start up has been completed.

The change is not critical, but it would be nice to have in 8.1.

Why is it important?

Reduces memory footprint of the beat.

Checklist

• My code follows the style guidelines of this project
  - [ ] I have commented my code, particularly in hard-to-understand areas
  - [ ] I have made corresponding changes to the documentation
  - [ ] I have made corresponding change to the default configuration files
  - [ ] I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

• Note that the same action happens as a deferred operation in the install path below this change when the temp file is removed.

beats/packetbeat/beater/install_npcap.go

Lines 70 to 77 in d74e7aa

	defer func() {
	// The init sequence duplicates the embedded binary.
	// Get rid of the part we can. The remainder is in
	// the packetbeat text section as a string.
	npcap.Installer = nil
	// Remove the installer from the file system.
	os.RemoveAll(tmp)
	}()

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[efd6]

/test

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• Start Time: 2022-02-22T21:32:02.563+0000

• Duration: 85 min 41 sec

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Test	Results
Failed	0
Passed	3328
Skipped	36
Total	3364

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