[debugger][dap] Disable integration test on riscv64

The dap_integration_test requires debug_agent to run.
However, debug_agent is not supported/implemented on riscv64
and is defined as an empty group in debug_agent/BUILD.gn.

A recent change (fxr/1723470) made the sibling test dap_symbol_unittest
reachable on all architectures, which caused GN to parse and validate
dap_integration_test on riscv64 builds, leading to a validation error
about missing debug_agent package manifest.

This change explicitly excludes dap_integration_test from the build
on riscv64, restoring the build to a working state (since the test
could never run on riscv64 anyway).

TAG: agy
CONV: 02d326b9-95f1-4296-afd2-f896f493bc5c
Change-Id: I6adc4b6edb068872e6daa5ebfd0c68bd6331cb9a
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1749649
Reviewed-by: Jacob Rutherford <jruthe@google.com>
Fuchsia-Auto-Submit: Nick Van der Auwermeulen <nickvander@google.com>
Commit-Queue: Nick Van der Auwermeulen <nickvander@google.com>