Update cheatsheets

tldr/linux/systemctl-set-environment

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+---
+syntax: markdown
+tags: [tldr, linux]
+source: https://github.com/tldr-pages/tldr.git
+---
+# systemctl set-environment
+
+> Set one or more service manager environment variables.
+> More information: <https://www.freedesktop.org/software/systemd/man/latest/systemctl.html#set-environment%20VARIABLE=VALUE%E2%80%A6>.
+
+- Set a single environment variable:
+
+`systemctl set-environment {{var value}}`
+
+- Set multiple environment variables at once:
+
+`systemctl set-environment {{var1 value1 var2 value2 ...}}`
+
+- Set an environment variable for the user service manager:
+
+`systemctl --user set-environment {{var value}}`