Add custom launchd log - mac unified logging sucks

While trying to help people for whom the launchd job failed, we found it
very difficult to find the entries in the new(ish) MacOS unified logging
thing, so update the launchd job to write output to a temporary log file.

The file is intentionally saved in /tmp and overwritten each time the
launchd job runs to avoid having to deal with permissions or rotation.

File name matches the plist name, e.g. homebrew.mxcl.aws-rotate-iam-keys.log

Formula/aws-rotate-iam-keys.rb

@@ -57,8 +57,12 @@ def plist; <<~EOS
       <array>
         <string>/bin/bash</string>
         <string>-c</string>
-        <string>( cat ~/.aws-rotate-iam-keys 2>/dev/null || cat #{etc}/aws-rotate-iam-keys ) | while read line; do aws-rotate-iam-keys $line; done</string>
+        <string>cp /dev/null /tmp/#{plist_name}.log ; ( cat ~/.aws-rotate-iam-keys 2>/dev/null || cat #{etc}/aws-rotate-iam-keys ) | while read line; do aws-rotate-iam-keys $line; done</string>
       </array>
+      <key>StandardOutPath</key>
+      <string>/tmp/#{plist_name}.log</string>
+      <key>StandardErrorPath</key>
+      <string>/tmp/#{plist_name}.log</string>
       <key>RunAtLoad</key>
       <true/>
       <key>StartCalendarInterval</key>

README.md

@@ -204,7 +204,12 @@ aws iam list-access-keys --profile default
 ```
 
 If it hasn't worked, check the MacOS system log for error entries matching
-`aws-rotate-iam-keys`.
+`aws-rotate-iam-keys`. If you can't find anything useful, the launchd job also
+writes output to a file in the `/tmp` directory matching the job name, e.g.
+
+```
+/tmp/homebrew.mxcl.aws-rotate-iam-keys.log
+```
 
 ### Other Linux
 

README.template.md

@@ -204,7 +204,12 @@ aws iam list-access-keys --profile default
 ```
 
 If it hasn't worked, check the MacOS system log for error entries matching
-`aws-rotate-iam-keys`.
+`aws-rotate-iam-keys`. If you can't find anything useful, the launchd job also
+writes output to a file in the `/tmp` directory matching the job name, e.g.
+
+```
+/tmp/homebrew.mxcl.aws-rotate-iam-keys.log
+```
 
 ### Other Linux