fix: prevent race condition when marshalling cached operations

[grevych]

What this PR does / why we need it

Re-bootstrapping with a recent version flags a race condition when running tests:

 :: Running go test (or_test)


  60ms . ·······················↷·································✖✖✖✖✖✖
       graphql_test

 63 tests, 1 skipped, 6 failures in 8.945s

=== Skipped
=== SKIP: . TestDoCustom (0.00s)
    do_test.go:51:

=== Failed
=== FAIL: . TestCustomOperationWithHeaders (0.01s)
    testing.go:1152: race detected during execution of test

=== FAIL: . TestQuery (0.01s)
    testing.go:1152: race detected during execution of test

=== FAIL: . TestCustomOperation (0.01s)
    testing.go:1152: race detected during execution of test

=== FAIL: . TestQueryWithHeaders (0.01s)
    testing.go:1152: race detected during execution of test

=== FAIL: . TestMutate (0.01s)
    testing.go:1152: race detected during execution of test

=== FAIL: . TestMutateWithHeaders (0.01s)
    testing.go:1152: race detected during execution of test

DONE 63 tests, 1 skipped, 6 failures in 8.963s
make: *** [test] Error 1

The issue occurs in the marshal method when multiple requests try to update the declaration name of the same cached operation https://github.com/getoutreach/goql/blob/6f72c1efa60eb971ac68dd8dbd602d133083d6e0/query.go#L636

The proposed fix copies the cached operation by value before it is updated. Since no other fields are updated during the marshalling process, there's no need make a deep copy (by reference).

Jira ID

N/A

Notes for your reviewers