CI: Use `nproc` to count only on-line CPUs for GNUMAKEFLAGS

Use `nproc` rather than `nproc --all`. Because the number by the `nproc` is
different from the number by the `nproc --all` on GitHub Actions ppc64le/s390x.
This caused the `make` command runs much more jobs than the number of on-line
CPUs on these environments.
The make command ran 193 jobs in parallel for 4 on-line CPUs in GitHub Actions
ppc64le, and ran 9 jobs in parallel for 4 on-line CPUs in GitHub Actions s390x.

And this caused the high load average 34.58 on ppc64le, and 6.69 on s390x.
These values should be less than 4.0. I believe we should use the `nproc`
rather than `nproc --all`.

```
+ nproc
4
+ nproc --all
192
```

```
+ nproc
4
+ nproc --all
8
```

See https://github.com/IBM/actionspz/issues/38 for details.

Note the `--all` option in the `nproc --all` was originally added to boost CPU
in a hyper threading environment where one physical core works like two logical
cores.

https://www.intel.com/content/www/us/en/gaming/resources/hyper-threading.html

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