Fix a problem with extra build commands. (#14528)

When --compileOnly is on, extraCmds are not added to JsonBuildInstructions.

```
$ echo 'echo 123'>test.nim
$ nim c --debuginfo --nimcache:cache test
$ ls
cache test test.dSYM test.nim
$ tail -n3 cache/test.json
"extraCmds": ["dsymutil /Users/khronos31/src/tmp/test"],
"stdinInput": false
}
```

```
$ echo 'echo 123'>test.nim
$ nim c --debuginfo --nimcache:cache --compileOnly test
$ nim jsonscript --nimcache:cache test
$ ls
cache test test.nim
$ tail -n3 cache/test.json
"extraCmds": [],
"stdinInput": false
}
```

0 files changed, 0 insertions, 0 deletions