"runInteractiveProcess: exec: does not exist" when invoking cabal with a relative path (OpenBSD)

On OpenBSD, with cabal-install 1.24, invoking the binary with a relative path gives an error:

```
$ .cabal/bin/cabal install parallel
Resolving dependencies...
Configuring parallel-3.2.1.0...
Failed to install parallel-3.2.1.0
Build log ( /home/maxim/.cabal/logs/parallel-3.2.1.0.log ):
cabal: Entering directory '/tmp/cabal-tmp-22828/parallel-3.2.1.0'
cabal: Leaving directory '/tmp/cabal-tmp-22828/parallel-3.2.1.0'
cabal: Error: some packages failed to install:
parallel-3.2.1.0 failed during the configure step. The exception was:
.cabal/bin/cabal: runProcess: runInteractiveProcess: exec: does not exist (No
such file or directory)

```

With an absolute path it's fine:

```
$ ~/.cabal/bin/cabal install parallel
Resolving dependencies...
Configuring parallel-3.2.1.0...
Building parallel-3.2.1.0...
Installed parallel-3.2.1.0

$ cabal --version
cabal-install version 1.24.0.0
compiled using version 1.24.0.0 of the Cabal library

```

I could reproduce this issue only on OpenBSD. On Mac OS it seems to be fine. Also, 1.22 doesn't suffer from this.