GH Actions: ensure that all versions are treated as strings

The CI job building the Perl 5.40 image failed, because `5.40` in YAML
was interpreted as a float and converted to `"5.4"`. To avoid these
problems in the future, let's secure all version numbers that YAML now
interprets as floats/integers.

.github/workflows/build.yml

@@ -29,9 +29,9 @@ jobs:
           - target: csharp
             implementation: net6.0
           - target: graphviz
-            implementation: 9
+            implementation: '9'
           - target: go
-            implementation: 1.22
+            implementation: '1.22'
           - target: java
             implementation: zulu7
           - target: java
@@ -53,31 +53,31 @@ jobs:
           - target: javascript
             implementation: nodejs20
           - target: julia
-            implementation: 1.9
+            implementation: '1.9'
           - target: lua
-            implementation: 5.3
+            implementation: '5.3'
           - target: nim
-            implementation: 1.6.0
+            implementation: '1.6.0'
           - target: perl
-            implementation: 5.24
+            implementation: '5.24'
           - target: perl
-            implementation: 5.40
+            implementation: '5.40'
           - target: php
-            implementation: 7.1
+            implementation: '7.1'
           - target: php
-            implementation: 8.3
+            implementation: '8.3'
           - target: python
-            implementation: 2.7
+            implementation: '2.7'
           - target: python
-            implementation: 3.4
+            implementation: '3.4'
           - target: python
-            implementation: 3.12
+            implementation: '3.12'
           - target: ruby
-            implementation: 1.9
+            implementation: '1.9'
           - target: ruby
-            implementation: 2.3
+            implementation: '2.3'
           - target: ruby
-            implementation: 3.3
+            implementation: '3.3'
 
     steps:
       - name: Dump matrix context