Using `replace-literal` to insert secrets leaks the secrets through
the `replace-literal` process' `/proc/<pid>/cmdline`
file. `replace-secret` solves this by reading the secret straight from
the file instead, which also simplifies the code a bit.
Bash doesn't handle subshell errors properly if the result is used as
input to a command. To cause the services to fail when the files can't
be read, we need to assign the value to a variable, then export it
separately.
