Logs do not exist

In #2909 and #2964 parallel Cabal attempts to print the build log to give users an idea what went wrong; however, something has gone wrong and the build log does not actually exist. Representative error:

```
$ cabal install cabal-install
Resolving dependencies...
Configuring cabal-install-1.22.6.0...
Failed to install cabal-install-1.22.6.0
Build log ( /Users/Lucas/.cabal/logs/cabal-install-1.22.6.0.log ):
cabal: /Users/Lucas/.cabal/logs/cabal-install-1.22.6.0.log: does not exist
```

It should be investigated why this is happening.

Start by reproducing the `cabal install cabal-install` failure shown in the issue and review the parallel log-printing attempts discussed in #2909 and #2964. Trace why the reported build-log path has no file, then verify that representative failed installs produce an available build log.

Written by the indexing model from the issue text.