How to target ldc compiler only in dub

[data pulverizer via Digitalmars-d-learn]

Hi,

I am trying to build a package to target LDC compiler only. I 
have both dmd and ldc2 (1.18.0) installed on my system. The dub 
file is:

```
{
        "authors": [
                "Me"
        ],
        "copyright": "Copyright © 2020, Me",
        "dependencies": {
                "mir-algorithm": "~>3.8.12",
                "mir-random": "~>2.2.14"
        },
        "description": "Some Cool Stuff",
        "license": "MIT",
        "name": "myPackage",
        "dflags": [
                "-O", "--release", "--boundscheck=off",
                "--ffast-math", "-mcpu=native"],
        "toolchainRequirements": {
                "dmd": "no",
                "gdc": "no",
                "ldc": ">=1.18.0"
        },
        "targetType": "executable"
}
```

I get the message:

```
Installed dmd 2.090.1 is not supported by myPackage. Supported 
compiler(s):
  - ldc: >=1.18.0
```

Thanks