indexer-cli throws java.nio.file.FileAlreadyExistsException on re-run with same index dir

### Affected version

7.1.6

### Bug description

When running the CLI with:

```shell
java -jar indexer-cli-7.1.6-cli.jar \
--name clojars \
--type min \
--index ./index \
--destination ./repo/.index \
--repository ./repo \
--checksums \
--quiet \
--chunks \
--keep 50
```

It runs successfully on the first run (when `./index` is empty). On a subsequent run, it fails with:

```
java.io.IOException: Error scanning context clojars: java.nio.file.FileAlreadyExistsException: /tmp/clojars-tmp10757666929368582455/nexus-maven-repository-index-packer.properties
at org.apache.maven.index.DefaultNexusIndexer.scan(DefaultNexusIndexer.java:313)
at org.apache.maven.index.DefaultNexusIndexer.scan(DefaultNexusIndexer.java:258)
at org.apache.maven.index.cli.NexusIndexerCli.index(NexusIndexerCli.java:448)
at org.apache.maven.index.cli.NexusIndexerCli.execute(NexusIndexerCli.java:164)
at org.apache.maven.index.cli.NexusIndexerCli.main(NexusIndexerCli.java:116)
Caused by: java.nio.file.FileAlreadyExistsException: /tmp/clojars-tmp10757666929368582455/nexus-maven-repository-index-packer.properties
at java.base/sun.nio.fs.UnixException.translateToIOException(UnixException.java:94)
at java.base/sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:106)
at java.base/sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:111)
at java.base/sun.nio.fs.UnixFileSystemProvider.newFileChannel(UnixFileSystemProvider.java:213)
at java.base/sun.nio.fs.UnixFileSystemProvider.newByteChannel(UnixFileSystemProvider.java:244)
at java.base/java.nio.file.spi.FileSystemProvider.newOutputStream(FileSystemProvider.java:426)
at java.base/java.nio.file.Files.newOutputStream(Files.java:215)
at org.apache.lucene.store.FSDirectory$FSIndexOutput.(FSDirectory.java:394)
at org.apache.lucene.store.FSDirectory$FSIndexOutput.(FSDirectory.java:387)
at org.apache.lucene.store.FSDirectory.createOutput(FSDirectory.java:220)
at org.apache.lucene.store.Directory.copyFrom(Directory.java:181)
at org.apache.maven.index.context.IndexUtils.copyFile(IndexUtils.java:73)
at org.apache.maven.index.context.IndexUtils.copyFile(IndexUtils.java:63)
at org.apache.maven.index.context.IndexUtils.copyDirectory(IndexUtils.java:56)
at org.apache.maven.index.DefaultNexusIndexer.scan(DefaultNexusIndexer.java:291)
... 4 more
```

Removing `./index/nexus-maven-repository-index-packer.properties` allows the subsequent run to succeed.

I encountered this when attempting to upgrade clojars.org from `4.1.2` to `7.1.6`.

Start at NexusIndexerCli.index and DefaultNexusIndexer.scan, following the IndexUtils.copyDirectory call shown in the stack trace. Reproduce the command with an existing ./index and inspect how nexus-maven-repository-index-packer.properties is copied on the second run. Done means repeated runs with the same index directory complete without requiring manual file removal.

Written by the indexing model from the issue text.