Add newline character to the end of file variable content

What does this MR do and why?

Text file should ends with a newline. how the POSIX standard defines a line:

A sequence of zero or more non- characters plus a terminating character

Therefore, lines not ending in a newline character aren't considered actual lines. That's why some programs have problems processing the last line of a file if it isn't newline terminated.

Relates to :#408413.

User can write content directly to a file,mostly user will forget "\n" since it's invisible in editor page, And Gitlab directly write it to file content which may

some ssh-key will have invalid format due to this issue

cc @daveliu

How to set up and validate locally

1. create a file that contains a line line1

   use cat -A(or cat -e in macOS) to check the file's content it will return

>cat -e test1
line1$

2. create a project.
3. jump to 'settings > ci_cd > Variables' and create a file variable, something like you can copy content from the file created in step1

4 create a gitlab-ci.yml to check the file content in pipeline

stages:
build

build:
  stage: build
  script: cat -A "$ROW"

it returns

which was different than step 1

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.