Fix race in ld.config.txt tests

The tests can run in parallel, and if they do, one test can truncate
ld.config.txt while another test is reading it (via the loader). Fix the
issue by using a TemporaryFile as the LD_CONFIG_FILE.

Test: adb shell /data/nativetest64/bionic-unit-tests/bionic-unit-tests  --gtest_filter=dl.exec_with_ld_config_file:dl.exec_with_ld_config_file_with_ld_preload:dl.disable_ld_config_file --gtest_repeat=1000
Bug: b/77555708
Change-Id: I9763caa076ece30d1a0eb9c8892a310ac51543b6