perf tests: Fix flakiness in trace record and replay test

The `perf trace record and replay` test fails intermittently on slow or
virtualized hosts because the default recording workload (`sleep 1`)
occasionally completes without scheduling the target `nanosleep` or
`clock_nanosleep` system calls inside the recorded sample window,
resulting in the error: `Failed: cannot find *nanosleep syscall`.

Generalize the `perf_record_with_retry` helper in
`tests/shell/lib/perf_record.sh` to support a custom record command prefix
via the `PERF_RECORD_CMD` environment variable (defaulting to "perf
record").

Update `trace_record_replay.sh` to use this robust retry loop running with
`PERF_RECORD_CMD="perf trace record"` and a base workload of `sleep`. The
test will automatically retry with scaled sleep durations (from 0.01s up
to 2.0s) until the required `nanosleep` event is successfully captured.

Fixes: 15bcfb96d0dd ("perf test: Add trace record and replay test")
Assisted-by: Antigravity:gemini-3.1-pro
Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

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