Fix nightly build links in issue forms

The issue forms are configured to request the contributor to test using the nightly build of Arduino IDE before
submitting an issue in order to make sure the bug or feature request has not already been resolved.

Some time ago, the repository's readme contained a table of download links. The links in the issue forms pointed there.
That table was replaced with a link to the official "Software" page in order to reduce unnecessary verbosity and
maintenance burden of the project's documentation content.

The issue form links were not updated at that time. The resulting additional link in the chain made obtaining the
nightly build less convenient for the contributor to obtain.

The links are hereby updated to point directly to the list of nightly build download links on the arduino.cc "Software"
page.

.github/ISSUE_TEMPLATE/bug-report.yml

@@ -30,7 +30,7 @@ body:
       description: |
         Which version of the Arduino IDE are you using?
         See **Help > About Arduino IDE** in the Arduino IDE menus (**Arduino IDE > About Arduino IDE** on macOS).
-        This should be the latest [nightly build](https://github.com/arduino/arduino-ide#nightly-builds).
+        This should be the latest [nightly build](https://www.arduino.cc/en/software#nightly-builds).
     validations:
       required: true
   - type: dropdown
@@ -68,7 +68,7 @@ body:
       options:
         - label: I searched for previous reports in [the issue tracker](https://github.com/arduino/arduino-ide/issues?q=)
           required: true
-        - label: I verified the problem still occurs when using the latest [nightly build](https://github.com/arduino/arduino-ide#nightly-builds)
+        - label: I verified the problem still occurs when using the latest [nightly build](https://www.arduino.cc/en/software#nightly-builds)
           required: true
         - label: My report contains all necessary details
           required: true

.github/ISSUE_TEMPLATE/feature-request.yml

@@ -25,7 +25,7 @@ body:
       description: |
         Which version of the Arduino IDE are you using?
         See **Help > About Arduino IDE** in the Arduino IDE menus (**Arduino IDE > About Arduino IDE** on macOS).
-        This should be the latest [nightly build](https://github.com/arduino/arduino-ide#nightly-builds).
+        This should be the latest [nightly build](https://www.arduino.cc/en/software#nightly-builds).
     validations:
       required: true
   - type: dropdown
@@ -63,7 +63,7 @@ body:
       options:
         - label: I searched for previous requests in [the issue tracker](https://github.com/arduino/arduino-ide/issues?q=)
           required: true
-        - label: I verified the feature was still missing when using the latest [nightly build](https://github.com/arduino/arduino-ide#nightly-builds)
+        - label: I verified the feature was still missing when using the latest [nightly build](https://www.arduino.cc/en/software#nightly-builds)
           required: true
         - label: My request contains all necessary details
           required: true