Fix AppVeyor env var value conditions

I assumed that the documented values of `true` and `false` were
booleans, not strings.

scripts/appveyor/build_installer.ps1

@@ -1,7 +1,7 @@
 
 $innoInstallPath = 'C:\Program Files (x86)\Inno Setup 5'
 
-if ($env:APPVEYOR_REPO_TAG) {
+if ($env:APPVEYOR_REPO_TAG -eq 'true') {
   if (!(Test-Path $innoInstallPath)) {
     choco install -y InnoSetup
 

scripts/appveyor/deploy_docs.ps1

@@ -19,7 +19,7 @@ $docsFiles = @(
 
 $commitMessage = 'Add v' + $env:APPVEYOR_REPO_TAG_NAME + ' release docs'
 
-if ($env:APPVEYOR_REPO_TAG) {
+if ($env:APPVEYOR_REPO_TAG -eq 'true') {
   cd 'C:\projects'
   git clone $docsRepoUrl