Backup .circleci/config.yml before regenerating (#54345)
Summary:
If you accidentally modify `.circleci/config.yml` directly and then run `.circleci/regenerate.sh`, it clobbers your changes. This PR saves the previous contents of `.circleci/config.yml` to a temporary file, whose name is printed to the console due to the `-x` already present in the script.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54345
Test Plan:
```
$ echo "418 I'm a teapot" > .circleci/config.yml
$ .circleci/regenerate.sh
```
Before:
```
++ dirname .circleci/regenerate.sh
+ cd .circleci
++ mktemp
+ NEW_FILE=/var/folders/vw/ryb6j4d97xs1t_14024b710h0000gn/T/tmp.vW7yBQT2
+ ./generate_config_yml.py
+ cp /var/folders/vw/ryb6j4d97xs1t_14024b710h0000gn/T/tmp.vW7yBQT2 config.yml
```
```
$ echo ':('
:(
```
After:
```
++ dirname .circleci/regenerate.sh
+ cd .circleci
++ mktemp
+ OLD_FILE=/var/folders/vw/ryb6j4d97xs1t_14024b710h0000gn/T/tmp.54GhUh7w
+ cp config.yml /var/folders/vw/ryb6j4d97xs1t_14024b710h0000gn/T/tmp.54GhUh7w
++ mktemp
+ NEW_FILE=/var/folders/vw/ryb6j4d97xs1t_14024b710h0000gn/T/tmp.aV87RTvQ
+ ./generate_config_yml.py
+ cp /var/folders/vw/ryb6j4d97xs1t_14024b710h0000gn/T/tmp.aV87RTvQ config.yml
```
```
$ cat /var/folders/vw/ryb6j4d97xs1t_14024b710h0000gn/T/tmp.54GhUh7w
418 I'm a teapot
$ echo ':D'
:D
```
Reviewed By: janeyx99
Differential Revision: D27195142
Pulled By: samestep
fbshipit-source-id: fcd9e4ac102ec3523d96f772eedbd42123364e26
