New tags from eval() node are not available in alert()

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The following TICK script does not work as expected because the new `foo` tag is not seen by the `alert()` node so the alert id is the empty string:
```
batch
|query('select ... as value from ...')
...
.groupBy('host')
|eval(lambda: ...)
.as('foo')
.tags('foo')
.keep('value')
|alert()
.id('{{ index .Tags "foo" }}')
...
```

OTOH, adding a dummy `groupBy()` node makes the `foo` tag available and the alert id is now correct:
```
batch
|query('select ... as value from ...')
...
.groupBy('host')
|eval(lambda: ...)
.as('foo')
.tags('foo')
.keep('value')
|groupBy('host', 'foo')
|alert()
.id('{{ index .Tags "foo" }}')
...
```

This has been seen with Kapacitor 1.5.0.

Start by reproducing the TICK script from the issue in Kapacitor 1.5.0, comparing the eval-to-alert pipeline with and without the extra groupBy('host', 'foo') node. Trace how eval().tags('foo') and alert() access tags, then verify that the alert id contains foo without the dummy groupBy node.

Written by the indexing model from the issue text.