Don't set thread name on main thread

[msimberg]

#1290 set the pthread name for all threads that are used by pika, including the main thread. Since we don't create or "own" the main thread, this PR removes setting the name for the main thread. E.g. in GDB the main thread will simply inherit the name of the binary, which I think is a lot more accurate.

For example, hello_world in pika_main:

(gdb) info threads
  Id   Target Id                                        Frame
  1    Thread 0x7ffff4efb9c0 (LWP 711869) "hello_world" 0x00007ffff4b1739d in __clone3 () from /nix/store/3bvxjkkmwlymr0fssczhgi39c3aj1l7i-glibc-2.40-36/lib/libc.so.6
  2    Thread 0x7ffff14ff6c0 (LWP 711952) "pika/w/d/0"  0x000055555596d8d3 in std::vector<pika::concurrency::detail::cache_aligned_data<pika::threads::detail::thread_queue<std::mutex, pika::threads::detail::lockfree_fifo, pika::threads::detail::lockfree_fifo, pika::threads::detail::lockfree_fifo>*, std::integral_constant<bool, true> >, std::allocator<pika::concurrency::detail::cache_aligned_data<pika::threads::detail::thread_queue<std::mutex, pika::threads::detail::lockfree_fifo, pika::threads::detail::lockfree_fifo, pika::threads::detail::lockfree_fifo>*, std::integral_constant<bool, true> > > >::operator[] (this=0x51e000000b78, __n=3)
    at /nix/store/v7dbfh5n7az2lcap0z1cv4jq0bikya8p-gcc-13.3.0/include/c++/13.3.0/bits/stl_vector.h:1129
* 3    Thread 0x7ffff0cfe6c0 (LWP 711953) "pika/w/d/1"  pika_main () at /home/mjs/src/pika/examples/quickstart/hello_world.cpp:19
  4    Thread 0x7fffef9f46c0 (LWP 711954) "pika/w/d/2"  0x00005555558cb86e in std::atomic<pika::threads::scheduler_mode>::load (this=0x51e000000088, __m=std::memory_order::relaxed)
    at /nix/store/v7dbfh5n7az2lcap0z1cv4jq0bikya8p-gcc-13.3.0/include/c++/13.3.0/atomic:297
  5    Thread 0x7fffee6ea6c0 (LWP 711955) "pika/w/d/3"  0x00005555555f57ec in __asan_stack_malloc_3 ()

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ -0.02% (target: -1.00%)	✅ 94.74% (target: 90.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (c8275ee)	18263	13784	75.48%
Head commit (73dfc2c)	18263 (+0)	13780 (-4)	75.45% (-0.02%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#1329)	19	18	94.74%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[biddisco]

Seems reasonable