ci: Clarify why valgrind task has gui disabled

A build with system libs (or with a normal depends build) will fail
with:

```sh
$ valgrind --exit-on-first-error=yes --error-exitcode=1 --quiet ./bld-cmake/bin/test_bitcoin-qt
Detected locale "C" with character encoding "ANSI_X3.4-1968", which is not UTF-8.
Qt depends on a UTF-8 locale, and has switched to "C.UTF-8" instead.
If this causes problems, reconfigure your locale. See the locale(1) manual
for more information.
********* Start testing of AppTests *********
Config: Using QtTest library 6.10.2, Qt 6.10.2 (x86_64-little_endian-lp64 shared (dynamic) release build; by GCC 15.2.0), ubuntu 26.04
PASS   : AppTests::initTestCase()
QINFO  : AppTests::appTests() Backing up GUI settings to "/tmp/test_common bitcoin/60d474ffae390f81657d/regtest/guisettings.ini.bak"
==18007== Conditional jump or move depends on uninitialised value(s)
==18007==    at 0x12655E26: ???
==18007==    by 0xCB28E7F: ???
==18007==
==18007==
==18007== Exit program on first error (--exit-on-first-error=yes)
```

A DEBUG=1 depends build would work, but that seems tedious for
questionable benefit.

ci/test/00_setup_env_native_valgrind.sh

@@ -15,7 +15,7 @@ export NO_DEPENDS=1
 # bind tests excluded for now, see https://github.com/bitcoin/bitcoin/issues/17765#issuecomment-602068547
 export TEST_RUNNER_EXTRA="--exclude rpc_bind --exclude feature_bind_extra"
 export GOAL="install"
-# TODO enable GUI
+# GUI disabled, because it only passes with a DEBUG=1 depends build
 export BITCOIN_CONFIG="\
   --preset=dev-mode \
  -DBUILD_GUI=OFF \