Fix test-app issue due to binary Hashicorp GPG key

[benhoyt]

Not sure how this ever worked -- maybe the Hashicorp URL used to return an ASCII key? Now it definitely returns binary, so use the gpg command to convert it to ASCII format.

___________________ test_install_package_external_repository ___________________
Traceback (most recent call last):
  File "/ops-libs-test/tests/integration/test_apt.py", line 45, in test_install_package_external_repository
    key = urlopen("https://apt.releases.hashicorp.com/gpg").read().decode()
UnicodeDecodeError: 'utf-8' codec can't decode byte 0x99 in position 0: invalid start byte

[benhoyt]

Hmmm, integration tests still failed. Seems like there's more to do here. For another day:

___________________ test_install_package_external_repository ___________________
Traceback (most recent call last):
  File "/ops-libs-test/tests/integration/test_apt.py", line 61, in test_install_package_external_repository
    repo.import_key(key)
  File "/ops-libs-test/lib/charms/operator_libs_linux/v0/apt.py", line 1056, in import_key
    self._write_apt_gpg_keyfile(key_name=key, key_material=key_gpg)
  File "/ops-libs-test/lib/charms/operator_libs_linux/v0/apt.py", line 1152, in _write_apt_gpg_keyfile
    with open(key_name, "wb") as keyf:
FileNotFoundError: [Errno 2] No such file or directory: ''
------------------------------ Captured log call -------------------------------
INFO     charms.operator_libs_linux.v0.apt:apt.py:1230 parsed 22 apt package repositories
WARNING  charms.operator_libs_linux.v0.apt:apt.py:1042 PGP key found (looks like Radix64 format). SECURELY importing PGP key from keyserver; full key not provided.
________________ test_list_file_generation_external_repository _________________
Traceback (most recent call last):
  File "/ops-libs-test/tests/integration/test_apt.py", line 85, in test_list_file_generation_external_repository
    apt.update()
  File "/ops-libs-test/lib/charms/operator_libs_linux/v0/apt.py", line 841, in update
    check_call(["apt-get", "update"], stderr=PIPE, stdout=PIPE)
  File "/usr/lib/python3.8/subprocess.py", line 364, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['apt-get', 'update']' returned non-zero exit status 100.
------------------------------ Captured log call -------------------------------
INFO     charms.operator_libs_linux.v0.apt:apt.py:1230 parsed 22 apt package repositories
INFO     charms.operator_libs_linux.v0.apt:apt.py:1230 parsed 1 apt package repositories