fix(loadt): Fix a bug that overwrote the original error

[mi-wada]

What I did in this PR

Currently at runn loadt cmd, the existing error (err) was overwritten by the new error returned from runn.RemoveCacheDir(). This change uses errors.Join(err, ...) to merge both errors and ensure the original context is not lost.

Background

By this bug, even if the condition of --threshold flag does not meets, no error was returned (exit with 0).

before

$ go run cmd/runn/main.go loadt --env-file load_test/.env load_test/main.yaml --duration 1s --warm-up 0s --threshold 'total > 10'

Number of runbooks per RunN....: 1
Warm up time (--warm-up).......: 0s
Duration (--duration)..........: 1s
Concurrent (--load-concurrent).: 1
Max RunN per second (--max-rps): 1

Total..........................: 2
Succeeded......................: 2
Failed.........................: 0
Error rate.....................: 0%
RunN per second................: 2
Latency .......................: max=21.0ms min=11.2ms avg=16.1ms med=11.2ms p(90)=11.2ms p(99)=11.2ms

after

$ go run cmd/runn/main.go loadt --env-file load_test/.env load_test/main.yaml --duration 1s --warm-up 0s --threshold 'total > 10'

Number of runbooks per RunN....: 1
Warm up time (--warm-up).......: 0s
Duration (--duration)..........: 1s
Concurrent (--load-concurrent).: 1
Max RunN per second (--max-rps): 1

Total..........................: 2
Succeeded......................: 2
Failed.........................: 0
Error rate.....................: 0%
RunN per second................: 2
Latency .......................: max=54.4ms min=16.0ms avg=35.2ms med=16.0ms p(90)=16.0ms p(99)=16.0ms

Error: (total > 10) is not true
total > 10
│
├── total => 2
└── 10

exit status 1

[k1LoW]

@mi-wada Thank you!!