BF: Add NSIS path to GITHUB_PATH for bash steps

When using bash shell in GitHub Actions on Windows, chocolatey's PATH
updates aren't reflected in the current bash session. Use PowerShell
for the install step and explicitly add NSIS path to GITHUB_PATH so
subsequent bash steps can find makensis.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: yarikoptic

.github/workflows/build-windows.yaml

@@ -135,7 +135,10 @@ jobs:
         run: stack install --no-haddock --local-bin-path .
 
       - name: Install NSIS
-        run: choco install nsis -y
+        shell: pwsh
+        run: |
+          choco install nsis -y
+          echo "C:\Program Files (x86)\NSIS" | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
 
       - name: Build the installer
         run: |

.github/workflows/template/build-{{ostype}}.yaml.j2

@@ -257,6 +257,12 @@ jobs:
       - name: Build git-annex
         run: stack install --no-haddock --local-bin-path .
 
+      - name: Install NSIS
+        shell: pwsh
+        run: |
+          choco install nsis -y
+          echo "C:\Program Files (x86)\NSIS" | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
+
       - name: Build the installer
         run: |
           stack ghc --no-haddock --package nsis Build/NullSoftInstaller.hs