Test SFDX release candidate every week

[jstvz]

This PR extracts the SFDX integration tests into a reusable workflow so that we can test against the SFDX CLI's release candidates. It is intended to provide early warning of breaking changes.

[jstvz]

Diffed output of sfdx plugins --core for the new workflow:

diff --git a/plugins b/plugins
index f47c954..6a0ec12 100644
--- a/plugins
+++ b/plugins
@@ -1,30 +1,29 @@
-@oclif/plugin-autocomplete 2.1.2 (core)
+@oclif/plugin-autocomplete 2.1.3 (core)
 @oclif/plugin-commands 2.2.10 (core)
-@oclif/plugin-help 5.2.5 (core)
-@oclif/plugin-not-found 2.3.20 (core)
+@oclif/plugin-help 5.2.6 (core)
+@oclif/plugin-not-found 2.3.21 (core)
 @oclif/plugin-plugins 2.3.2 (core)
 @oclif/plugin-search 0.0.13 (core)
-@oclif/plugin-update 3.1.5 (core)
+@oclif/plugin-update 3.1.6 (core)
 @oclif/plugin-version 1.2.1 (core)
-@oclif/plugin-warn-if-update-available 2.0.28 (core)
+@oclif/plugin-warn-if-update-available 2.0.29 (core)
 @oclif/plugin-which 2.2.16 (core)
 @salesforce/sfdx-plugin-lwc-test 1.0.1 (core)
-alias 2.2.2 (core)
-apex 2.2.0 (core)
-auth 2.7.1 (core)
+apex 2.2.1 (core)
+auth 2.7.3 (core)
 community 2.2.3 (core)
-config 1.5.1 (core)
-custom-metadata 2.1.3 (core)
-data 2.3.1 (core)
-info 2.4.4 (core)
+custom-metadata 2.1.5 (core)
+data 2.3.4 (core)
+info 2.4.7 (core)
 limits 2.3.5 (core)
-org 2.4.2 (core)
+org 2.4.4 (core)
 packaging 1.15.0 (core)
 schema 2.3.1 (core)
-sfdx-cli 7.190.2 (core)
-signups 1.4.3 (core)
-source 2.5.4 (core)
+settings 1.4.0 (core)
+sfdx-cli 7.191.1 (core)
+signups 1.4.5 (core)
+source 2.5.5 (core)
 telemetry 2.1.1 (core)
-templates 55.4.1 (core)
-trust 2.4.2 (core)
+templates 55.4.4 (core)
+trust 2.4.4 (core)
 user 2.3.2 (core)