Fix (nearly) all issues found by shellcheck

shellcheck found more issues than SC1074. Address most of these issues.
You can check the shell code by running:

```
shellcheck -e SC1090,SC1091 -s dash hook-functions $(find * -type f
\( -executable ! -name rules -o -regex '.*\.\(post\|pre\).*'
-o -regex "^\(docs\|scripts\)/.*" ! -name '*.md' \))
```

Signed-off-by: Benjamin Drung <benjamin.drung@cloud.ionos.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/hooks/thermal b/hooks/thermal
index 93f5100..5807386 100755
--- a/hooks/thermal
+++ b/hooks/thermal
@@ -28,7 +28,7 @@ powerpc|ppc64)
 
 	[ -r /proc/cpuinfo ] || exit 0
 
-	MODEL="`grep model /proc/cpuinfo`"
+	MODEL="$(grep model /proc/cpuinfo)"
 	MODEL="${MODEL##*: }"
 
 	case "$MODEL" in