docs: simplify Linux sudo instructions in README

[jnm]

Describe an easier way to pass the SENTRY_IMAGE environment variable to install.sh when using sudo that doesn't require modifying the sudo configuration.

Demonstration

Simple test script:

jnm@world$ cat > install.sh
#!/usr/bin/env bash
echo "It looks like you are $(whoami) and want to install $SENTRY_IMAGE"

jnm@world$ chmod +x install.sh

Basic environment variable test:

jnm@world$ SENTRY_IMAGE=whee ./install.sh
It looks like you are jnm and want to install whee

Oops, sudo doesn't forward the variable from its own environment by default (#922):

jnm@world$ SENTRY_IMAGE=whee sudo ./install.sh
[sudo] password for jnm:
It looks like you are root and want to install

However, there's an easier way than modifying the sudo configuration:

jnm@world$ sudo SENTRY_IMAGE=whee ./install.sh
It looks like you are root and want to install whee

My environment is Ubuntu 20.04 (with the KDE neon desktop environment):

jnm@world$ sudo --version
Sudo version 1.8.31
Sudoers policy plugin version 1.8.31
Sudoers file grammar version 46
Sudoers I/O plugin version 1.8.31

jnm@world$ lsb_release -a
No LSB modules are available.
Distributor ID: Neon
Description:    KDE neon User Edition 5.22
Release:        20.04
Codename:       focal

This method is also described in the same StackOverflow discussion that was referenced by #922. Thanks for your consideration, and thanks for making Sentry available for on-premise installations!

[chadwhitacre]

Thanks for the PR, @jnm! Looks good at a skim, give me a day to think about it a bit more and also invite @BYK's input to make sure there's no real reason we were doing it the other way. 🤔

[BYK]

Great simplification, thanks a lot!

[BYK]

Will wait till @chadwhitacre and @scefali to chime in before merging.

[scefali]

@BYK I have no opinion on this as long as the README is accurate

[chadwhitacre]

Nice simplification. Thanks @jnm!

[BYK]

Sorry, did not realize we forgot to merge this in 🙂