Swiftly installation instructions are inaccurate for shells like fish

[MahdiBM]

Describe the bug

https://www.swift.org/install/macos/
instructs to use this command to install swiftly:

curl -O https://download.swift.org/swiftly/darwin/swiftly.pkg && \
installer -pkg swiftly.pkg -target CurrentUserHomeDirectory && \
~/.swiftly/bin/swiftly init --quiet-shell-followup && \
. ~/.swiftly/env.sh && \
hash -r

However, for the fish shell the second-to-last line should be . ~/.swiftly/env.fish && \

The current instructions results in this error:

Once swiftly is installed it will install the latest available swift toolchain.

Proceed? (Y/n):
y
Creating shell environment file for the user...
Updating profile...
Fetching the latest stable Swift release...
Installing Swift 6.1.0
                                                   Downloading Swift 6.1.0
100% [========================================================================================================================]
Downloaded 1462.2 MiB of 1462.2 MiB

Installing package in user home directory...
The global default toolchain has been set to `Swift 6.1.0`
Swift 6.1.0 installed successfully!
.: Error encountered while sourcing file '/Users/my-user/.swiftly/env.sh':
.: No such file or directory

You can still manually run source /Users/my-user/.swiftly/env.fish.

To Reproduce

Steps to reproduce the behavior:

Use fish shell and follow the installation instructions.

Expected behavior

Smooth installation for users of other shells.

Actual behavior

Errors as described.

Screenshots

Environment details

Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

If possible, minimal yet complete reproducer code (or link to code)

Swift.org version/commit hash

Additional context

[MahdiBM]

Maybe the second to last line can be changed to . ~/.swiftly/env.$(basename $SHELL) && \?

I'm not 100% sure this'll behave correctly with Swiftly, but i think basename is available everywhere so using that should be fine.

[MahdiBM]

cc @cmcgee1024 since this is swiftly related.

[cmcgee1024]

See #946