[v11] web: Ignore .swc directory when computing web SHA

[camscale]

Ignore any .swc directories when computing the SHA of SHAs to
determine if make ensure-webassets should rebuild the web UI. The
.swc directories are in the .gitignore file, so should also be
ignored when computing the SHA of the web files.

On a fresh checkout of teleport, running make ensure-webassets
causes a plugin to be build or downloaded into
web/packages/teleport/.swc/plugins/v4. As this is inside the directory
over which the SHA of SHAs is computed, if you re-run
make ensure-webassets, it ends up rebuilding the web UI for the same
result. It should not rebuild the web UI if it hasn't changed. The SHA
of SHAs generated from a fresh checkout of teleport should match another
fresh checkout. This fails as generating the enterprise webassets after
generating the OSS webassets includes the plugin as part of the SHA, and
that is not there on a fresh checkin.

This will make a difference if we want to build the web assets as a
separate step on CI so that the webassets directory can be copied into
other builds. This will allow a later version of node.js to be used to
build the web UI that what may be available on the OS we're building
Teleport on (I'm looking at you, Centos 7).

Fix a shellcheck-reported issue of quoting while we're here.

Also (partial) backport some previous PRs that made minor changes to
build.assets/build-webassets-if-changed.sh.

Issue: #21724
Backport: #29892
Backport: #21822
Backport: #22979 (partial)
Changelog: none

[camscale]

v12 doesn't use Vite so the SWC directory doesn't get created (#29898 (comment))

Thanks for that. I wasn't sure, but I checked the .gitignore and saw it had the Vite section for .swc. I'll close this.
(I'm assuming if v12 does not use Vite, neither does v11).