Build failure happens when one tries to build `nix` on a linux
system as user with later plan to install as root (typical package
building separation):

```
$ cd /tmp
$ git clone https://github.com/NixOS/nix.git
$ cd nix
$ ./bootstrap.sh
$ ./configure --prefix=/usr ac_cv_header_aws_s3_S3Client_h=no
$ make
...
$ LANG=C make
  LD     /usr/lib/libnixutil.so
/usr/lib/gcc/x86_64-pc-linux-gnu/12.0.0/../../../../x86_64-pc-linux-gnu/bin/ld: cannot open output file /usr/lib/libnixutil.so: Permission denied
collect2: error: ld returned 1 exit status
make: *** [mk/lib.mk:117: /usr/lib/libnixutil.so] Error 1
```

Build failure happens because `doc/manual/nix.json` depends
on installed `$(bindir)` path instead of local inplace path:

```
$ make -r -d doc/manual/nix.json
...
Considering target file 'doc/manual/nix.json'.
 File 'doc/manual/nix.json' does not exist.
  Considering target file '/usr/bin/nix'.
    Considering target file 'src/build-remote/build-remote.o'.
     Looking for an implicit rule for 'src/build-remote/build-remote.o'.
...
doc/manual/local.mk:$(d)/nix.json: $(bindir)/nix
```

The fix is to always use inplace built `src/nix/nix` binary.

Closes: #5184