quote some variables, and use double-quotes for more readable output

[thaJeztah]

• relates to https://github.com/docker/docker-install/pull/443

I noticed some variables weren't quoted but included parts that could be set through environment variables.

Add quotes to those to prevent possible issues, and use double quotes to make the output more readable, similar to what was done in commit 65cca2fde3a7fe72cd6a2d498bfd86c5ff4a9ba1

Before:

+ TMP_REPO_FILE=/tmp/tmp.gg8DOiskXT
+ sh -c 'curl -fsSL '\''https://download.docker.com/linux/fedora/docker-ce.repo'\'' | tr -s '\''\n'\'' > '\''/tmp/tmp.gg8DOiskXT'\'''
+ sh -c 'dnf5 config-manager addrepo --save-filename=docker-ce.repo --overwrite --from-repofile='\''/tmp/tmp.gg8DOiskXT'\'''
+ sh -c 'rm -f '\''/tmp/tmp.gg8DOiskXT'\'''

After:

+ TMP_REPO_FILE=/tmp/tmp.C89hwiQBFk
+ sh -c 'curl -fsSL "https://download.docker.com/linux/fedora/docker-ce.repo" | tr -s "\n" > "/tmp/tmp.C89hwiQBFk"'
+ sh -c 'dnf5 config-manager addrepo --save-filename=docker-ce.repo --overwrite --from-repofile="/tmp/tmp.C89hwiQBFk"'
+ sh -c 'rm -f "/tmp/tmp.C89hwiQBFk"'