[github/templates] Add samples cmds to get OS env (#7550)

* [github/templates] Add samples cmds to get OS env

When reporting bugs, users doesn't seem to know what information to
report for the line "OS / Environment".

- `uname -a`: Provide the arch + kernel (which has the dist tag name)
- `grep -Hv ^# /etc/*release`: most of distribution provide some sort
of /etc/*release file. Adding -H to get the filename too. And -v ^#
to remove comments if any

Example on my Fedora:

```
uname -a; grep -Hv ^# /etc/*release
Linux munin 5.2.18-100.fc29.x86_64 #1 SMP Tue Oct 1 13:32:05 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
/etc/fedora-release:Fedora release 29 (Twenty Nine)
/etc/os-release:NAME=Fedora
/etc/os-release:VERSION="29 (Twenty Nine)"
/etc/os-release:ID=fedora
/etc/os-release:VERSION_ID=29
/etc/os-release:VERSION_CODENAME=""
/etc/os-release:PLATFORM_ID="platform:f29"
/etc/os-release:PRETTY_NAME="Fedora 29 (Twenty Nine)"
/etc/os-release:ANSI_COLOR="0;34"
/etc/os-release:LOGO=fedora-logo-icon
/etc/os-release:CPE_NAME="cpe:/o:fedoraproject:fedora:29"
/etc/os-release:HOME_URL="https://fedoraproject.org/"
/etc/os-release:DOCUMENTATION_URL="https://docs.fedoraproject.org/en-US/fedora/f29/system-administrators-guide/"
/etc/os-release:SUPPORT_URL="https://fedoraproject.org/wiki/Communicating_and_getting_help"
/etc/os-release:BUG_REPORT_URL="https://bugzilla.redhat.com/"
/etc/os-release:REDHAT_BUGZILLA_PRODUCT="Fedora"
/etc/os-release:REDHAT_BUGZILLA_PRODUCT_VERSION=29
/etc/os-release:REDHAT_SUPPORT_PRODUCT="Fedora"
/etc/os-release:REDHAT_SUPPORT_PRODUCT_VERSION=29
/etc/os-release:PRIVACY_POLICY_URL="https://fedoraproject.org/wiki/Legal:PrivacyPolicy"
/etc/redhat-release:Fedora release 29 (Twenty Nine)
/etc/system-release:Fedora release 29 (Twenty Nine)
```

* Update command to have MacOS and Unix working too

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