Fix false error with storeDir and val outputs

[bentsherman]

Close #5785

This PR fixes a false positive error with storeDir that causes it to be overly strict with value outputs.

Currently storeDir only works with value outputs of the form val(<name>), where <name> is defined as an input or in the script block. However, any value output should be allowed, because value outputs are always evaluated and do not depend on the storeDir or resume cache.

Test case:

process FOO {
    storeDir 'store'

    output:
    tuple val(task.process), val('grep'), path('version.txt')

    script:
    """
    grep --version | head -n 1 > version.txt
    """
}

workflow {
    FOO()
}

Currently, the two value outputs above will caused the task to not be stored. With this PR, the task is stored correctly.

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[jorgee]

It would be good to add the test case as integration test

[bentsherman]

TODO: update dynamic-storedir.nf to include this case