fix: ensure init.d-style install sees our default environment variables

[danxmoran]

Addresses #20132

I think the set trick will work since the shebang on the script points at /bin/bash, let me know if it's not portable.

Tested by building the package & installing into a fresh ubuntu VM. Without these changes, I see this in the logs:

ts=2020-12-15T22:52:12.224551Z lvl=info msg="Resources opened" log_id=0R69W2FG000 service=bolt path=/var/lib/influxdb/.influxdbv2/influxd.bolt

After this change, I see:

ts=2020-12-15T22:56:05.258672Z lvl=info msg="Resources opened" log_id=0R69jGY0000 service=bolt path=/var/lib/influxdb/influxd.bolt

With these env / config file contents:

$ cat /etc/default/influxdb2
INFLUXD_CONFIG_PATH=/etc/influxdb/config.toml

$ cat /etc/influxdb/config.toml
bolt-path = "/var/lib/influxdb/influxd.bolt"
engine-path = "/var/lib/influxdb/engine"

[psteinbachs]

I think this will be fine and shouldn't pose any portability issues.