When running `nix build -L` it can be fairly hard to read the output if
the build program intentionally renders whitespace on the left. A
typical example is `g++` displaying compilation errors.

With this patch, the whitespace on the left is retained to make the log
more readable:

```
foo> no configure script, doing nothing
foo> building
foo> foobar.cc: In function 'int main()':
foo> foobar.cc:5:5: error: 'wrong_func' was not declared in this scope
foo>     5 |     wrong_func(1);
foo>       |     ^~~~~~~~~~
error: --- Error ------------------------------------------------------------------------------------- nix
error: --- Error --- nix-daemon
builder for '/nix/store/i1q76cw6cyh91raaqg5p5isd1l2x6rx2-foo-1.0.drv' failed with exit code 1
```

libmain/progress-bar: don't trim whitespace on the left