Fix output index file write to object storage

[bentsherman]

Closes #7633

Problem

Writing a CSV index file from an output block fails when the output directory is a gs:// bucket:

java.lang.UnsupportedOperationException: APPEND
	at com.google.cloud.storage.contrib.nio.CloudStorageFileSystemProvider.newWriteChannel(CloudStorageFileSystemProvider.java:461)
	at org.apache.groovy.nio.extensions.NioExtensions.leftShift(NioExtensions.java:500)
	at nextflow.util.CsvWriter.apply(CsvWriter.groovy:43)
	at nextflow.extension.PublishOp.onComplete(PublishOp.groovy:236)

CsvWriter.apply() built the file with Groovy's path << text operator, which opens the file with StandardOpenOption.APPEND. GCS objects are immutable once written, so its NIO provider rejects that option outright.

The issue was reported against google-batch, but the executor is irrelevant. The index file is written by the head job, so any run with a gs:// output directory hits this, local executor included. Setting index.path to a local path does not help either, since PublishOp.onComplete() always resolves it against session.outputDir.

Only the CSV branch was affected. JSON and YAML index files already assign path.text = ..., a single write.

Change

Build the CSV in a StringBuilder and write it once. Index files are a list of published outputs, small enough that buffering costs nothing.

Two lines dropped as redundant: the leading path.delete() (assigning path.text truncates) and the if( records.isEmpty() ) path << '' case (assigning an empty string already creates the file).

Testing

CsvWriterTest passes unchanged, including the empty-file case and the test that writes the same path twice.

Verified against a live GCS bucket. Before the change the run aborted with the stack trace above; after it, the object is written and reads back correctly:

"id","value"
"sample_1","1"
"sample_2","2"
"sample_3","3"

Note that CsvWriterTest uses a Jimfs in-memory filesystem, which permits append, so the existing tests could never have caught this. A regression test would need a filesystem that rejects APPEND. I have left that out of this PR, happy to add it if reviewers want it.

Unrelated observation

The failed run printed [SUCCESS] and exited 0 despite onComplete throwing, so a broken index write is silent in CI. Separate issue, not addressed here.

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